Beat the game, finish the RR episode, catch 'em all (barring Zeraora), what to do now? Any elusive side quests? Bonus trainer battles? Otherwise I could be doing a shitload of Mantine surfing.
Battle Tree or Battle Royal. They can be used so you can get BP, used for buying many amazing items. Try and truly catch all eight hundred something Pokemon, maybe? Wormhole hunt? Lots of various options.
Ultra wormwhole shiny hunting :D