>>33114642USM is alternate universe, and that's basically the only way they could have new alolan forms by virtue of how they're treating them like IRL evolution, so I'd say that's practically guaranteed. ORAS introduced new megas after X and Y and since there's probably no new types, new Z-moves are basically out the window except for two possibilities: giving each type a physical and special variant Z-Move because things like All Out Pummeling and Corkscrew Crash don't exactly fit when you use them from Focus Blast or Flash Cannon, or giving more pokemon exclusive Z-Moves, which could be cool depending on who gets it. I'd like to see stuff like Heatmor getting one because it got a new signature move in Fire Lash, or Ariados' new Toxic Thread. This seems unlikely based on being compatible with Sun and Moon, but since there's already a new Solgaleo and Lunala form not found in Sun and Moon when datamined, it's still possible. Speaking of that, there's a decent amount of pokemon who got new bells and whistles like Mantine getting Roost and Bellossom getting Quiver Dance, but not being found in Sun and Moon, so you may be able to find different mons in USM and try out their new tricks in game.
Anyway, with new alolan forms, we'll likely get non-gen 1 stuff simply because so many Gen 1 pokemon have gotten either an evolution, mega, or alolan form at this point and they need to keep some untouched so they can do new stuff with them down the line to cash in on gen 1 pandering.
While no info is definitely a curse, it can also be a blessing in some way. We can't predict what wasn't datamined from Sun and Moon, but we know Solgaleo and Lunala's new forms weren't there, so there's still hope of being surprised.
It's a matter of perspective, really. Try to look on the positive side, OP.
