Quoted By:
ITT: Ideas for the next Pokemon game.
Mine:
Pokemon Eclipse
Takes place a year after Sun and Moon.
Lusamine has awoken from her coma, but she can't remember anything, much to Lillie's distraught.
Chaos awakens and ripples through Alola when Marshadow awakens (trapped inside a cocoon that is Necrozma), and a portal to another dimension opens. The portal connects the Sun-version universe to the Moon-version universe. The opening makes the unstable nature of Ultra Beasts from each dimension mysteriously merge together.
Buzzwole + Guzzlord
Kartana + Pherosma
Celesteela + Nihlego + Xurtree
Solgaleo + Lunala (they create Marshadow)
The Pokemon League is completed and the champion is both Sun and Moon, of whom you face in a double battle with your rival, Erika.
The reason Alolan 'mon are so slow is because Marshadow's cocoon sucks the life force from the entire region of Alola, which in place damages the stats of the 'mon. Alola 'mon get fixed stats in this game, and transferring up through Bank or trading adjusts the stats appropriately.
The Battle Tree has new members, and it is revealed the Battle Tree was made just a little bit before Sun and Moon takes place ("built a year and a half ago") so there weren't that many opponents.