Just a quick list of the top 3 Pokemon of each generation I want Ash with (even if he already has them); Cross generation evolutions will be when the line was first introduced:
1st Generation-
1. Dragonite- He already has it,
2. Gengar- Might be getting it, so its to the list anyway
3. Rhyhorn-Rhyperior (maybe at least a Rhydon)- Would be a nice catch to make up for the 30 Tauros he got
2nd generation
1. Larvitar-Tyranitar- Most likely to return and be caught, especially if its THE Larvitar
2. Espeon- My favorite Eeveelution and the one I want him to have the most.
3. Togepi-Togetic-Togekiss- To make up for the fact that it should've been Ash's Togepi to Togekiss
3rd Generation
1. Ralts likely to Gallade- I love Gardevoir more than Gallade, but I have to be reasonable here and say they would likely give him Gallade over Gardevoir.
2. Spoink- Ash has bonded with Spoink before (I even still remember when 3rd generation was around I erroneously thought Ash did have a Spoink, might have more than likely confused it with Snorunt), Grumpig would be a nice addition
3. Trapinch/Flygon- This is what I want but I doubt it'll happen.
4th generation:
1. Riolu-Lucario- This seems pretty likely and I hope it happens
2. Gible-Gabite-Garchomp- Look, I know Dragonite is making it difficult to imagine Ash with more dragon types, but just evolve Gible into Garchomp, so he can have it please.
3. Skorupi/Drapion- Just because I really like Drapion and think it would be a cool team member for Ash.
5th Generation:
1. Chandelure- Going with the most likely option, it'll be guaranteed if Ash catches Litwick
2. Golurk- I WISH, but probably not.
3. Cubchoo-Beartic- Because I wanted it really badly in 5th gen since I had to accept defeat about Ash
Evolutions instead- Palpitoad (Because Ash should have Seismitoad), Gigalith (because Ash was robbed of a cool Pokemon, even though Trip was allowed to have Conkeldurr and thus completely ruined a good story between them), and finally Snivy into Servine (which is something she deserved at least since they insisted on not doing anything at ALL since revealing leaf blade, at least a 100 episodes of NO CHANGE, and to hell with Trip for forcing Snivy to remain unevolved by opening his yap about Snivy not being evolved.
6th Generation-
1. Phantump- That's absolutely it.
7th Generation-
1. Golisopod- I'll go with this
8th Generation-
1. Impidimp-Grimmsnarl- It's my 8th generation Golett, Phantump and Jangmo-o because of that it's not going to happen, but I want it to happen.
2. Corviknight- Just because I think its the most badass regional bird ever.
3. Toxel-Toxtricity- Just give Ash another electric type, please. It's dual typed, so it shouldn't cause Pikachu suffering. Besides it could be nod to Naganadel.
4. Grookey- I think we can live without Sobble, might be too much like Greninja, and Ash doesn't have a Rillaboom like Pokemon yet (at least not of his grass Pokemon)
True, what would we count as "ones that got away"? Would they be exclusively Pokemon ash attempted to capture that he didn't get like dunsparce and rhyhorn, or would they include Pokemon ash bonded with but never caught like houndour and venipede?
I think this primarily depends on at least one factor:
1. How many Pokemon is Ash going to get? If it's going to be more than Unova (or Kanto) does this mean Ash will catch Pokemon that serve no ultimate purpose, like will Ash be a mini Go (just in far less quantity of course), or will we be guaranteed that Ash will heavily use these Pokemon.
2. (minor factor) Will they at leas try to focus on other generations (of Ash at least getting 1 from each generation).
Like I can imagine Ash potentially getting a Rhyhorn fully evolving into Rhyperior and being used as a strong Pokemon, but that'll be another Kanto Pokemon.
Dunsparce would tick off the 2nd generation but wouldn't it just be considered a Pokemon Ash has that might get a battle or two but is ultimately at Sakuragi lab for the rest of the series.
Houndour? Sure will it evolve into Houndoom, and will we see a Scolipede in Ash's future?