I think that they are going to make the Ultra Space a lot bigger than just a few random places like Nihilego's was in SM. At this point, I get the feeling that Nihilego's US was put into SM as a teaser for USUM. I get the feeling that it is going to be a completely explorable place much like the Distortion World, but on a grander scale. Heck, for all we know a large chunk of the story is going to take place in the Ultra Space and not in the actual Alola Region (especially with the absolute lack of information for these games when they are only three months away, and compared to SM information we had at this point to release last year, I feel like they are saving something extremely huge for these games. Maybe they listened to fans and are keeping the huge thing a secret and only teasing bits and pieces of it instead of outright giving us all the major information. That cloud/portal thing seems to important to just be random hallways with UB's at the end, then again, it might end up being another soaring in the sky thing with Hoopa's portals from ORAS). It could be that it truly is a new region for us to explore, and while it may not be (and probably isn't) as large as Alola (or other regions), it could be comparable to Johto like Alola is to Kanto. It might give them a reason to include new Alolan Forms of Gen II Pokémon. But that is all clearly speculation. The biggest thing I'm worried about (right now) is whether to use Dusk Form or Midnight Form Lycanroc.
EDIT: where is the new Japanese trailer? I can't seem to find it anywhere.
EDIT 2: I'm also really hoping that the dex includes more older Pokémon. I really want to use a Tyranitar on my team in this game. And even if the dex isn't expanded, I hope trade is available fairly early in the game so I can trade a Larvitar over.