Besides the pokedex, battle frontier, and E4 rematches, what should I do? I beat every quest on the wiki page, and captured most of the legendaries.
if you’re interested in trying another run (since you can have up to 3 save files), why not try a challenge run? randomizers and egglockes are really fun in my experience :))
There is also the Living Dex or Alphabetic Shiny Hunting (having one shiny for every letter of the alphabet using the first letter of their name.)