Honestly, Ultra Sun/Moon really wasted the only return I would honestly really have cared about; Cyrus.
The end of Pearl/Diamond/Platinum was so open-ended and good.. as opposed to most games, Cyrus, albeit being defeated, did not give in nor abandon his ideals.. instead, he was trapped within the distortion world, still vowing to remove spirit from the Pokemon world. His re-emergence, if handled well, would possibly be such a huge narrative moment. And yes, while USUM Cyrus was an alternate dimension one, it still somewhat lessens the shock/surprise if he emerged in such a way that established him as an antagonist.