So, consider this:
Everyone is expecting either SM 3rd Version/sequels or DP Remakes (my personal pick). A few people think it may be another Kanto game, more speficically, Red/Green re-remakes or sequels.
Maybe these people are partially right.
The 20th movie is a retelling of the first episodes of the original series. While it features newer mons like Piplup and Lucario which do feel as a hint to DP remakes, or Incineroar, a gen 7 mon, the movie also features Charizard and Butterfree which were part of Ash's og team, and it's still a very Kanto-based movie.
Now, the movie celebrates the 20 years of the anime, and what game was made specifically to tie-in with the anime? Yellow
Yellow's official english name is "Pokémon Yellow Version: Special Pikachu Edition" and in Japan it is simply called "Pocket Monsters: Pikachu". It features elements of the anime in the main story (like Jessie and James, and the player getting all 3 Kanto starters).
Another point is while there's certainly references to Sinnoh and unfinished stuff dealing with Necrozma, there's also a lot of references to Kanto, including Lillie going there at the end of the story.
Also, the last time Kanto was featured as the main region was in FRLG which are still older than DP.

So, basically, what I'm claiming is the next game could be a Yellow remake that retells the game's plot (like the 20th movie is doing with the OG series) and includes all the new stuff introduced in later games and maybe even continues SM's plot (Necrozma, reencountering Lillie, etc.).

I know the Virtual Console releases kinda kill this chance, but I think it's still something to think about.