not really a prediction, but an idea for a story for a Sun/Moon sequel:
It's some time after Sun/Moon (like let's say 1-5 years). The new player character is a new trial goer. Not far into the game (like maybe after clearing the first trial), the Ultra Beasts invade Alola again, except this time the invasion is massive. The combined forces of the Elite 4, trial captains, island guardians, Interpol, and whoever else happens to fight against them (like Guzma and Hau or something) are struggling to keep them at bay. Sometime thereafter, the player encounters an Ultra Beast and fights it off. Gladion witnesses the battle, and is reminded of his friend, the previous player character. Feeling that he can trust the player, he gives them the last Type Null that was kept in stasis and a few Beast Balls, asking them to help fight against the Ultra Beast invasion--he and the Aether Foundation have their hands too full coordinating rescue efforts, damage control, taking care of the injured, researching the UBs, etc, to directly combat them, and the previous player character has inexplicably vanished, and so humanity is without their most powerful weapon against the beasts (Alola's Champion and Solgaleo/Lunala).
Trials could be fit into the narrative as a way for the player to get stronger and become BFFs with Null so they can save Alola from the UBs. There could be a 3rd stage for the Type Null line introduced in the hypothetical sequel (basically they have a bronze-silver-gold theme going, but we don't have the gold part right now, just the bronze (Null) and silver (Silvally), not to mention it's based off of Arceus and Arceus's shiny is golden colored). The Ultra Beasts could finally have the backstory/lore they should've gotten in Sun/Moon, and they could introduce new UBs too (and have all of them in the game, no version exclusives this time). Could even squeeze Necrozma in there maybe (just not as the cover legendary, let's try a story this time that doesn't involve a climax that involves catching a legendary).
Single game, not as a pair. No villain team this time either.
/random thoughts