Guys, even if they are possibly just random, we should analyse the Pokemon that appear in the poster, as this could lead us to potential spoilers. Let's do it through regions:
1º) KANTO:
*Maybe I'm wrong, but I believe the building representing Kanto here could be the Indigo Plateau, where you can defy the Elite Four. After Ash winning Alola League, he certainly can win Kanto's League; then gaining a pass to defy Kanto's Elite 4.
+ Bulbasaur and Squirtle: I hope they are Ash's Pokemon, which could mean the return of Ash's Squirtle to his team.
+ Charizard and Charmander: Charizard could be Ash's Pokemon, but it really confuses me that Charmander on his back. (I don't think it will be his kid).
+ Mew: When it appeared in the logo, I suspected Mew would have an important role in this series. And what would you know?, I was right. We know Go's goal is finding and capturing this Pokemon. (Note to remember, Mew made a cameo on Oak's Lab during Alola's arc).
+ Articuno, Zapdos and Moltres: I suppose Ash and Go will have encounters with these three legendary Birds, probably in the same places where you can find them in the games. Articuno in the Seafoam Islands, Zapdos in the Power Plant and Moltres in Victory Road. (I doubt Ash or Go will capture any of them, although we know Ash's Charizard can defeat an Articuno).
+ Jigglypuff: I suppose our old friend, the singing Jigglypuff will return even after Alola.
+ Ditto: I guess this could be captured by Go, because if he plans to capture ALL Pokemon, then Ditto could be useful for him, as it transform into any Pokemon.
+ Dragonite: A Dragonite hugging Meowth while Wobbuffet looks? That's really weird. Is a Dragonite going to replace Bewear after Alola? That doesn't sound good.
+ Mr Mime: It could be Delia's Pokemon, but I wish Ash could add it somehow to his team.
+ Eevee: I believe this could be captured by Go, either to promote a new "Eeveelution" or simply because it can evolve to so many Pokemon, he can choose one.
+ Gengar: I always wanted for Ash to capture this Pokemon, especially after Haunter's fiasco. Maybe we will finally see it happen.
+ Snorlax: It could be Ash's Pokemon. But wit a minute, is that a "Dynamax/Gigantimax" form? A giant Snorlax would certainly be scary. It may be able to eat more than Guzzlord!!
2º) JOHTO:
*The Bell Tower clearly indicates we could explore again the mysterious link that appear to connect Ash with Ho Oh.
+ Lugia: We have a funny scene with Go and Ash riding on Lugia, although I don't get why Ash is in the tail is a "funny way". After all, he was the "Chosen One" back in the second movie. Now that I think of it, could this Lugia be Silver? You know, the baby Lugia Ash's team and Ritchie saved from Team Rocket.
+ Ho-Oh: Clearly we will explore the link between Ash and Ho-Oh, who seems to act as Ash's "Spirit Guide" in the Pokemon World.
+ Cyndaquil: We have Cyndaquill, but no Chikorita or Totodile. Clearly, this can't be Ash's Quilava, but I would like Ash using the three of them back for a time and made some of them evolve. Honestly, Feraligator is the one that suit Ash better, but he could evolve the three of them.
+ Togepi: It can't be the same Pokemon that belonged to Misty. Do you think Ash will find/capture another one? After all, he found the original Togepi's egg.
+ Unown: I can imagine Ash and Go being trapped into a crazy adventure with these Pokemon, similar to the third movie or something like that.
+ Sudowoodo: I don't think it will be a captured Pokemon. So, maybe we will see Brock back with his own Sudowoodo?
+ Magby and Smoochum: The return of the Pichu Brothers? These two were part of their gang in the short about their adventures.
+ Espeon and Umbreon: I would like to believe Ash could capture one of them, and Go could capture the other one. Since his old rival Gary had an Umbreon, Ash could capture an Espeon.
This si getting too long, so I will post the other regions later.