>>18469363>Battle Maison>Friend Safari>Mewtwo, Zygarde and the bird(s)>Looker's Quest>Whatever the highest level Battle Restaurant is>Megastone side quest>Shiny Charm for completing dex>Unlocks some in-game trades>Battle Spot ladderingIt does feel like it's missing something, but I don't know what I'd add. Certainly not more shitty pushover gym leaders which are just glorified Youngster Joeys though, even though that's what everybody seems to want when they say "postgame".
I think adding Challenge Cup, Little Cup and Inverse Battles to the Battle Spot would pretty much make up for any lack of postgame quests.