*Late-June(last week of June and so): So far we don't have the regional bug, rodent, bird, as well as Pikaclone, regional fish, and regional Fire, as the latter especially should be revealed around that period or early July. These guys are obviously worthless, but they might have rare or new type combinations, which would be important to note for. We also don't have the legendaries' types, signature moves and Abilities, which should be revealed soon-ish.
*July: Generally second month is when our early Fighting-type is revealed (Bewear in July, Pancham in June, Pangoro in July) and we see a fair amount of Fairies(Mimikyu and Comfey in July, Flabébé in June, Swirlix and Spritzee in July) but since we already have our early Fairy(Impidimp) we will likely see our early or mid-game Ghost instead(Mimikyu and Honedge were both revealed in July).
*August: This is when Z-Moves and Mega Evolution were announced. Third month is also when evil team and the animé series is generally announced(though that might be next month, see
September 2016), so there is going to be a lot here. This is also when VGCs will be in their finals, so we will get obligatory opening ceremony announcement here as well. Since cross-generational evolutions and regional variants are rumored to be coming back, this should be when we get the first taste of them.
*September: Fourth month generally doesn't have anything specific, but being historically the release month, new Pokémon tend to appear here. But overall nothing too big here. Maybe one pair of version exclusives?
*October: Penultimate month. This is generally reserved for evolutions, especially the starter ones. Penultimate month(September for XY) was also when Fossils were revealed, so chances are it will be the same here or the previous month. If the cross-generational evolutions are back, this will when they will open the floodgates with them(see August 2006).
*November: Game will go gold and will be leaked before the release. But in the recent years, third legendary has been shown in the release month or the month after. Being the release month, this will largely cover the post-game stuff, so there won't be much here.
Stuff remaining that doesn't have a specific release date for:
*Early Psychic: This tends to appear in Summer, but which month we won't know. Since this is an even generation, our early Psychic should be low BST two stager(see Natu/Mime Jr./Espurr for reference).
*Early Ground: Also tends to be Summer, generally August(see August 2016 for Mudsdale and August 2010 for Drilbur/Moguryu). Our early Ground is almost always two staged, so there isn't much to expect here, except for it to exist unlike Kalos, that is.
*Post-game facility: Tends to be revealed late, but generally revealed before the release(except for Kalos). Battle Tree was revealed in October, and that seems to be decent and recent enough reference date.
*Regional dog/cat: Not really an "archetype", but a common theme that happens. Regional cat/dog tend to fall into existing categories(Rockruff is Gen 7's early Rock-type, Purrloin is Gen 5's Poochyena/early Dark etc.) but they are marketable, so they appear early. We already Yapper as our early Electric(Pikachu/Mareep/Electrike/Shinx etc.), so chances are our regional cat will be either our early Dark or early Psychic again.
*Early Rock: This one is a wild one. Roggenrola was revealed in early September 2010(with Gigalith being revealed August 2010) and Rockruff was revealed in August 2016, so this can appear any time, probably sooner than later, though. But since we're most likely getting fossils, this might either not exist, or might be post-release reveal like Alolan Geodude, who shares the early Rock-type spot with Rockruff.
*Late Ice: This generally appears, except for Kalos, though in Kalos's case it did exist(Bergmite), we just didn't saw it early on. If there is a more mid-game Ice-type like Amaura, this probably won't appear.
*Late Dragon: This is another conundrum. While Noivern was introduced early on, Noibat's stats suggests it was meant to appear much earlier, and of course Alola famously broke the trend by revealing our pseudo so early. This is a "we'll wait and see", case.
*Various gimmick monsters(including form changing ones): Since most gimmicks tend to mimic existing archetypes, we can't say when they'll be revealed unless we know what they are. Other than they will exist, that we can say.
*Other rivals: Another somewhat inconsistent reveal. Most likely they will be revealed in June or September to avoid being drowned by August's infodump, but that's a huge time difference, so IDK.
*Regional mascot: In other words, the Arcanine/Absol/Lucario/Zoroark/Lycanroc of the generation. The cool aloof (generally) canid. Yamper's evolution might be it, given how it appears in the stadiums, but that is unlikely.
*One of the mid-game Gym Leaders: Ash needs travelling companions, which tend to be either early Gyms or mid-game ones(except famously Iris, who was merely the youngest). Since we already have what seems to be the first two, others should follow suit.
Other stuff that might appear if they exist:
*Secondary and tertiary bugs: Again, generic early route fodder. If they exist, we'll most likely see them.
*Mid-game elementals: In other words, mid-game Fire, Electric, Grass and Water. While we won't necessarily see all of them, some of them might appear, again, if they exist.
*Secondary early Grass: There doesn't seem to be any counterpart or colleague to Gossifleur like Lurantis was for Tsareena, but if there is, it will be revealed.
*Mid-game Fighting: Scraggy was revealed early, same goes for Croagunk, so if there is one, we'll see it.
*Secondary Bird: If Corviknight is our regional bird, that is. Otherwise, it already is this.
*Mid-game Steel: Again, if Corviknight is not our regional bird, then we will likely get another Steel for the Gyms 3-5. If there is no such Steel for early or mid-game, then it won't be revealed, unless it is the pseudo.
*Mid-game Dragon: While Kalos technically didn't have this(Tyrunt is a Fossil, Skrelp fills the regional fish quote alongside its version counterpart Clauncher) both Druddigon and Drampa/Turtonator were released early, though Drampa is technically found in the end-game unlike its counterpart.
*Late-game filler: Your Ursarings, your Donphans, your Trevenants etc. Stuff that appears in the final routes of the game, but have relatively generic types. Since we don't have Victory Road, such a thing might not necessarily exist, but if it does, we will see a few them.
*Evolutions and pre-evolutions of the revealed stuff: Besides the starters, we don't have a specific date for the evolutions of the pre-release mons, so you can go wild with the speculation. Generally speaking the evolutionary relatives appear about a month or two later, so you can fill in your wildest dreams for July. Obviously all of them are not guaranteed to appear, but in the last few gens they tend do so.
For reference, we already have:
*Generational gimmick (Dynamax)
*Early Fairy/Dark (Impidimp)
*Early Electric (Yamper)
*Early secondary Normal (Wooloo)
*Early terrestrial Water (Dreadnaw)
*???(Corviknight)
*One of the rivals (Hop)
*Professor and their assistant (Magnolia and Sonia)
*Starters (Scorbunny, Sobble, Grookey)
*Mascot legendaries (Zacian, Zamazenta)
*And the Champion (Leon)