Once you beat the demon in paper Mario 2 what do you do next?

Anything. You can attempt the Pit of !00 trials, solve trouble's at the trouble, make a comeback at the Glitz Pit(Not much different from the first time and not as fun. But that's just my opinion), etc. You can also see If you can find some of the old partners from the old paper Mario game, unfortunately there just for display and talk, they cant join your party :(