I'm in the 'these aren't sequels' camp, but I do think it's possible that some of the additional content could expand the story past the end of sumo, perhaps like the Delta Episode in ORAS. So, I think it "could" contain some sequel-ethnicity components, but not anywhere near a full story.
My theory/predictions of the game, which could be entirely wrong:
These are going to be packaged and play as an alternative story much like Emerald, Crystal, etc. I think the story will be surprisingly different, overall better then Sumo, but similar enough to warrant complaining from a large section of the internet (who bought it and played it anyway, despite knowing in advance because we're all suckers for Pokémon in the end). I think there will be some merit to the complaining though, as it will likely include some features that had no reason NOT to be in Sumo in the first place. It will have some neat new gimmick though that made sense to hold back on (probably something to do with the different shapes z-crystal Sun twists in the trailer briefly). There will be a few newly accessible areas to explore on the islands (the golf course, a safari zone-like park to catch fossil Pokémon in, etc). There will be another small island to explore, probably visited briefly during the campaign and visited in full post game where we find move tutors.
There will be no Gender bending Characters or inverting their personalities, but many might be in different situations. Instead of freshly moving to Alola, we will have been raised here for several years (perhaps even born there) and we won't need so much hand-holding introduction cut scenes to the idea of Alola and Trials because our character would be familiar with them. Guzma's trial captain/gym leader, and skull will be run by someone else, like Plumeria or Gladion. Lusamine will not be evil, and her husband Mohn will still be around and the family dynamic changed. I think the "evil team" is likely to be the ultra beasts themselves. Looker will feature more prominently, we might see Hau's father (potential champion?).
I think we will get new Alolan forms, perhaps another 15-20, of Johto and Kanto Pokémon that didn't appear in SuMo. We will also get more Ultra beasts, perhaps 5-7. We will get the national Dex this time, and the lack of one in SuMo will have something to do with compatibility issues with the new ultra beasts and alolan Pokémon. New exclusive z-crystals for Pokémon like Mymikyu, Togedemaru and Lycanroc. Our pokémon will follow us in the overworld.
There will be no direct mention of the SuMo story, but if you search in journals or other very side hints, we can find out/piece together that the sumo story happened in an alternate dimension that we actually sort of crossed paths with (via Mohn or something really subtle). YouTubers will make dozens of theory videos to cover it and pick it apart.
One other idea that I have is maybe instead of team skull, we're facing a sort of team of "traditionalists". The idea would be that Alola recently established gyms and an elite four, but many are outraged at the change and what it's done to the trial system Alola always had, so they cause trouble to try and disrupt/end the gyms. In game, you can do both gyms and trials and it plays as a sort of background to the Necrozma and Ultra Beast narrative. My issues with this idea though is that it doesn't connect well to the ultra beasts and legendary side of the story.
