Let's do a comprehensive list of what Ash could catch, why or why not (Not counting legendaries). *NOTE* This is all highly subjective. **NOTE** Forgive any mistakes, this was made mostly at the time of the leaks and lack of information in the Anime.
Why? Grookey is sort of a safe bet in comparison to the other starters. He's only ever had one missing Grass starter, Chespin, and thus it might more likely he'll get Grookey. And Ash could be seen with Thwackey or even Rillaboom.
Why Not? There is some debate over whether Ash will get a Galar starter this generation, we haven't seen anything to indicate as such. Since all starters are pure typing, the last time we had that was 2nd generation, therefore it leaves something to be desired.
Why- Scorbunny has safe evolutions, in that it wouldn't make sense not to fully evolve Scorbunny. Cinderace looks like a hero type Pokemon more so than the others, and would fit nicely.
Why not- Go. Ash is very unlikely to capture a Scorbunny because one already belongs to his traveling companion.
Why- Sobble probably has the most controversial evolution of all three. It might be interesting to see Ash with a spy themed chameleon.
Why not- Probably because it is has the most controversial evolution. The final form has received mix reception. While I do believe the anime will make it look much better, I think its ability to go to Ash will depend largely on whether or not it has any cool signature spy moves.
Why? Ash has typically always gotten the regional bird of the series with the one exception of the Alola region. But why? Because he had Rowlet which was a Grass/flying starter, thus it was not necessary for him to also get Pikipek. On top of that of all the Pokemon with flying types listed. Rookidee is the only one that stands out.
Why Not- There could always a chance Ash doesn't get the regional bird and not even a flying type starter (which there isn't), it's possible Ash might not get a flying type.
Why? Ash has never gotten the regional rodent before, so here's a chance to finally get a chubby one.
Why Not? Ash has never gotten the regional rodent before. LOL, I guess in this case it being pure normal doesn't stand much chance with Ash. If Ash was getting a normal type, it would need to likely stand out more.
Why- Of the normal types, Wooloo seems like it has the most likely option. Not necessarily a Pokemon that might stick with Ash, but who knows maybe a Pokemon he catches for the sake of catching.
Why Not- It's normal type is something to be less than desired. It honestly looks more like a companion Pokemon
Why? If Ash didn't get Grookey but had to get a Grass type, it's either this or something else. If they did give it to Ash, they might see it as a unique option.
Why Not? Gossifleur is a strange grass type, it almost comes off as the opposite of the Flabebe line, it looks like it would be a fairy type but its not, the reason why it wouldn't be caught is perhaps they think it would belong more as a non main Pokemon or maybe even a companion Pokemon.
Why? Nicket and its evolution always came off as a little weird for me. It's not the design I have a problem with in of itself. Objectively I really like Nicket's design, the problem is the 3D model makes Nicket look like it belongs to another franchise completely. HOWEVER the anime will obviously change it, since I can see it having a much more smooth transisition.
Why Not? While it is a dark type, and could work, I feel like its too much like the equivalent of Poochyena. Likelier to be a companion Pokemon, but I'm not against it.
Why? Yamper would give Ash a second electric type Pokemon.
Why Not? It's unlikely that it being given to Koharu would mean Ash would get it. It's pure typing and its evolution basically being just a dog (LMAO) kind of makes it seem unlikely. If Ash was going to get another electric type, it would have to be dual typing.
Why? Milcery and its evolution are fairy types and they would be interesting to give to Ash especially since Alchremie comes in many different flavors, it doesn't necessarily have to have the drawback of being feminine.
Why Not? The only real issue is that there are more suitable fairy types out there, and I feel like the sheer number of Alchremie forms might dissuade them from giving one to Ash. If it had a dual typing, I would be more likely to understand.
Why? This could be a very safe option. Ash could capture a Blipbug and evolve it into Ash's first psychic type. There are more fun psychic types, but it is a safe option.
Why Not? If Ash is going to get a Water Type, Grass Type and Fire Type, given the poor options available Blipbug might be replaced by a different Bug type, unless Ash gets more than one bug type.
Why? It would give Ash his first Pikaclone.
Why Not? Its a Pikaclone.
Why? If Ash doesn't get a water type in Sobble, the only other Pokemon he's left with is either this or one of the fossil combinations, as such this might be his only option.
Why Not? Ash has gotten turtle Pokemon before and this isn't a bad choice, it's just unclear if Drednaw (based on its Pokedex entry) would work for Ash if his goal is to travel everywhere.
Why? One of my favorite Pokemon revealed so far, especially of the psychic variety, though Orbeetle is pretty cool. Giving Ash a Hattena would give Ash a first psychic type that might actually work for Ash. The final evolution being part fairy is questionable but it still would be a good choice
Why Not? It's hard to know which feminine Pokemon work and which ones don't. I think of whether a Pokemon is designed for girls or for boys. Hattena is a weird one, since its almost like Raltz which boys do pick (Wally, Max, etc), but they may ultimately decide not to give it to Ash.
Why? Giving Ash a fish Pokemon for once, would actually be interesting, especially since it makes more sense to seldomly appear kind of like Kingler (but Kingler can actually battle)
Why Not? The risk of giving Ash a fish Pokemon and rarely using it might be why the wouldn't give Ash the Pokemon. But hey they gave him Palpitoad.
Why? Of the water types, one could make the argument this might work for a choice and its a flying type.
Why Not? It being a water/flying and a standalone might be enough to dissuade it from being caught by Ash alone. But add to that there's one other issue. Cramorant is more of a gag Pokemon. Sure some have been forced on Ash (though the species might not have been a gag Pokemon). It's unknown if gag and gimmicky Pokemon will end up on Ash's team. Gimmicky in context to its gulp missile ability.
Why? Silcobra is a nice ground type that might work really well with Ash. It's kind of like a Dunsparce/Snivy/Ground mix. In that it kind of resembles Dunsparce, relates to Snivy as a snake, and is a ground type.
Why Not? Team Rocket are going to get Pokemon as well, and since this does look like a burgular type Pokemon with Silcobra having what looks like a burgular sac, it probably will go to Team Rocket.
Why? Sinistea would definitely be an interesting ghost type Pokemon that Ash would catch. It would be his first.
Why Not? The Sinistea line looks way too much like it would fit Team Rocket more.
Why? A call back to Roggenrola but with a twist. Ash isn't going to get Scorbunny, so Carkoal and Coalossal might be a fair chance at a fire type.
Why Not? There's probably one better fire type. And if its like Roggenrola or Geodude, it'll need to be a trade evolution.
Why? Phanpy. Ash caught a Phanpy and so it would be cool to give Ash another elephant Pokemon, especially one that is a steel type. And Copperajah looks pretty neat. Donphan and it should get along fine.
Why Not? There isn't much reason, outside of perhaps its insane design might not translate too well in the anime, but we'll see.
Why? For the love of god, this is really the only Pokemon I desire on Ash's team the MOST. I need it to be owned by Ash, I need to fully evolve, I just need the michevious little Imp on Ash's team. Would be a great first fairy and its dark type would be great. It could be Ash's strongest 8th generation Pokemon
Why Not? While its not my opinion, I do have to accept that it might be too mischevious or too evil to be on Ash's team. But of all the Pokemon it is my favorite, and I do want it on Ash's team.
Why? Forget ALL other electric types, this is the one that I think has the highest chance of being caught by Ash as a secondary electric type. It's dual typing, its a poison type, and therefore would likely have a great shot.
Why Not? Alas there may never be a 2nd electric type on Ash's team. Though Ash did seem to have some interest in Zeraora and I don't think Pikachu would have a problem. Toxtricity looks more like a physical electric type anyway so that would contrast Pikachu has been more of a special electric attacker anyway.
Why? Snom is a little weird but dear GOD is Frosmoth one of the most magestic Pokemon revealed so far. It's ice bug typing would be a really nice Pokemon for Ash. Who doesn't have much experience in Ice (outside of Lapras/Glalie). So Frosmoth would be really cool on Ash's team. Arguably one of my favorite 8th generation bug types.
Why Not? Not the best bug for Ash this generation.
Why? Honestly the best choice for Ash if he's going to get a fire type. Centiscorch is honestly a really cool fire and bug type, that I think would ultimately work really well with Ash, especially if its going to be a big boy.
Why Not? While I do think its the best bug type for Ash since he's not getting Scorbunny. There are two other bug types that might fit better with Ash in terms of marketing or just simple ease. It all depends on what they want Ash to have. If Ash was going to get two I'd choose Frosmoth and Centiskorch, but I could live with Orbeetle with either of these options.
Why? For a fighting type, I do think Clobbopus has a much a higher chance of Ash getting it than he did Crabrawler. And Grapploct will be great with Hawlucha. It's a shame it's not a water type, otherwise that would be my top pick.
Why Not? It's pure typing is a little off putting, and Ash only has two of the only wrestler Pokemon (well if you consider Hariyama to be a little different and not the same as Hawlucha/Incineroar), so they might pull back on it, plus it would be 3 generations in a row.
Why? It's steel dragon typing would be an interesting choice for Ash to have. It's not like Ash would get a Dialga for example
Why Not? One of the weakest dragon types that could go to Ash. There are at least two better options
Why? Stonjourner is certaintly an interesting Pokemon that could be given to Ash. I mean it's unknown what purpose it would serve.
Why Not? It's inclusion on Ash's team would be weird in comparison to the other Pokemon he could get.
Why? Second Electric type?
Why Not? It's pure typing is left to be desired, but since Toxel was already poison type, I guess Pincurchin also being poison would've been weird and Toxel would've been superior choice anyway. Pincurchin might be a team rocket Pokemon.
Why? If it is a psychic dark type that would definitely be an interesting Pokemon to give to Ash.
Why Not? Indeedee very much looks like it'll either be a companion Pokemon, a rival Pokemon or even a Team Rocket Pokemon, which to me means Ash won't get it. It looks more like it'll be a rival's Pokemon in the sense that it might be like their starter or signature, it'll be their equivalent of a Pikachu.
Applin Line (?)
Why? A Grass/Dragon is definitely a cool typing, and if Ash doesn't get Grookey this Ash's best grass type choice he could get. Which is why I think Grookey is Ash's best choice here for marketing.
Why Not? Flapple makes sense, Appletun, just confuses the hell out of me, is it a split evolution or is it an evolution from Flapple. I ask because Flapple looks like it would have a specific direction, suddenly changing a winged grass dragon thing to a grounded giant dragon apple pie/bakery item, is extremely weird, but it could happen.
Why? Eiscue is ridiculous enough that it would be hilarious to give Ash this Pokemon.
Why Not? It might be too ridiculous.
Why? Not only is it a dragon type but its also a ghost type, this might be Ash's best chance to get a ghost type THIS generation of all the options since the other ghost types are Galarian forms (unless you think Sinistea has a chance).
Why? It's an interesting fighting type.
Why Not? It's standalone and gimmicky especially since it appears to be more than one thing.
Why? Same for all four of these Pokemon. Ash having one of the four options for a fossil types Makes sense.
Why Not? It's an electric dragon which is cool but seems incredibly unlikely of the four options.
Why? Same as Dracozolt, but I do like the Electric/Ice combination the best (although it doesn't have the best design).
Why Not? It is part electric, and I do think there are better electric types.
Why? Same as Dracozolt, but here's a little bit more likeliness. If Ash doesn't get Sobble, I do think Dracovish or Arctovish would be Ash's best chance.
Why Not? As cool as the dragon typing is, there are better dragon types.
Why? Same as Dracozolt but with added twist. I do think its water/ice type and its design would fit Ash the best, especially if he doesn't get Sobble.
Why Not? If Ash gets Sobble he might get something else and its unclear if these are the best choices. The weird fossil combination and their typings might also be a problem.
Why? While Cuffant are better steel types, it still would be interesting to have Ash with a Meowth.
Why Not? I feel like Go with Galarian Meowth would be much better choice.
Why? Pontya is probably my favorite regional variant of all of them.
Why Not? Out of all the Galarian forms its the least likely for multiple reasons. I don't think they would give Ash a My little Ponyta, and of all the Galarian forms it's one of the few that doesn't have anything unique except typing (it shares this with Weezing, Darumaka, and Stunfisk)
Why? Would definitely be interesting to see.
Why Not? It's probably my least favorite Galarian variant. Only because I'm not sure I understand Sirfetch'd all that much other than its a knight of some kind with a giant lance. If Ash is getting Corvinight I feel like giving Ash Sirfetch'd would be redundant. Plus I think if Ash gets Corviknight Sirfetch'd would work better as a rival Pokemon.
Why? IF Koffing is normal and evolves into Galarian Weezing then I doubt Ash will get one. Since I doubt they'd go through giving Ash a Koffing.
Why Not? I think Go catching a Koffing might present a more unique interaction especially with James from Team Rocket. Besides imagine Go catching Koffing and bringing it to Galar only to evolve it there.
Mr. Mime line
Why? Galarian Mr. Mime would be a unique choice for Ash, it would be psychic/ice. This stands out because I imagine it would get along with Delia's Mr. Mime.
Why Not? Well it might be too crazy to give Ash his own Mr. Mime.
Why? It definitely would be interesting to give Ash a ghost type Corsola. Maybe even have it meet Misty's Corsola in a future episode.
Why Not? It's pure ghost typing is weird especially since you'd think it would at least be part rock. How does a water/rock become pure ghost anyway. Like did it really become that ethereal?
Why? Ash getting a Stunfisk would be a nice call back to Cilan, and Ash might even interact with Cilan this generation. Ash with a Galarian form Stunfisk might definitely be interesting to see interacting with Cilan's.
Why Not? Go could just be more likely and give Go a reason to interact with Cilan, while Ash has history.
Why? It was the major Galarian form shown. I imagine it would be hilarious if Ash caught a Zigzagoon in Hoenn thinking it was shiny, taking it to Galar only to realize (WAIT, it's actually a Galarian Zigzagoon that just happened to be in the Hoenn Regions).
Why Not? Obstagoon really looks like it would belong more to a rival Pokemon than a Pokemon Ash actually has. Plus giving Ash something like Grimmsnarl might be a great counter
Why? It certaintly is a weird ice type galarian form.
Why Not? It reeks too much of Sandshrew and Vulpix, the insistence of not creating ice types but just using old Pokemon, the difference is at least 8th generation created more than one ice type Pokemon, so it does get a pass there.
Why? I really wanted Ash to potentially get Yamask, so there is a pretty nice chance here. It's ground typing could be pretty nice, providing a ghost/ground Pokemon.
Why Not? While Ghost/ground is a cool typing, I still feel iffy it makes sense to give Ash such a Pokemon like that. There are much better ghost types.