A Possible Return Of The Legendary/Mythical Pokemon In Pokemon USUM?

As in older regions' legendary Pokémon being catchable in the wild? Most likely not. There might be mentions of them in dialogue and such though, who knows.

 

I agree with Luxray, I don't see them returning outside of mentions. Of course they could appear in post-game like they did in ORAS but they will not be in the main game unless they affect the story in some way, and I'm pretty sure they'll save the story for Solgaleo, Lunala and Necrozma.

 

If that's the case with fusions, perhaps they'll make mention of Kyurem, as it's the only other Pokémon with that ability that I can think of.

 

It seemed like ORAS/XY was the time they were focusing on older Legendaries; they didn't show up too much in S/M. I would think that there would be numerous references, perhaps deliberately distorted (parallel universe and all), but I doubt they would appear.
#MissingNo

 

I highly doubt it to be honest. We have had all of the Mythicals last year and in ORAS you could find almost all of the other legendaries in the Post game. To do another massive distribution like that is not really needed nor feasible in the long run.