Hope you're right.
Just as Emerald and Platinum expanded the Battle Towers of RS and DP into Battle Frontiers, I suspect that USUM will expand SM's Battle Tree into a PWT equivalent. We'll likely get the rest of the Champions (Lance, Steven, Wallace, Alder, Iris, Diantha), and the presence of Grimsley and Anabel potentially opens the door to the other E4 Members and Frontier Brains (who had infamously been left out of the PWT).
Since Wally's there, we may also see other rivals, namely ones than can be included without canonizing a certain starter (so no Silver, Barry, or Hugh, but Cheren, Bianca, Shauna, Tierno, and Trevor are possible). Of course, protagonists besides Red are a no-go, but if these are sequels, they may let you see your old SM protagonist via a Memory Link-type feature.
I don't think we'll see any Gym Leaders, however, as SM seemed to go out of their way to not bring back any past Gym Leaders. This may end up being an alternate PWT that swaps out Gym Leaders in favor of E4 Members, Frontier Brains, rivals, and various other characters who had been excluded from the Gen 5 PWT.
Well, one look at the mascots should tell you that Necrozma will somehow be involved, and the "ultra" in the games' titles likely points to the Ultra Beasts playing a larger role, too. What remains to be seen is whether not these are sequels like B2W2 or another third version like Emerald/Platinum, although the wording of the Direct suggested the latter.
Looking further back, it's somewhat known that Yellow was released to buy GF time to finish up GS (which were suffering delays). Crystal was likely also put out to buy time for RS, while Emerald was almost certainly meant to be buy time for DP (although GF severely underestimated just how delayed DP would end up being).