I beat the game, got thundurus, what can I do now?
Try a challenge run on another save (like nuzlocke + hard mode + battle style SET), farm IV stones, hunt shinies, breed competitive pokemon, etc.
Or wait for 1.2.
You could use tesseract to find other hidden pokemon and items, collect the secret base flags to get the rewards, complete the pokedex, try for a living dex, breed competitive pokemon, improve/revamp your team for next update, hunt for shinies and/or start a challenge run, play a few of the other fangames out there, go on with your life and/or wait.
Thanks!