Link and Finn having a pokemon battle? (With Jake pretending to be a pokemon, since he can morph himself?) Lady Rainicorn, a Rapidash, and Epona? Princess Bubblegum/Marceline along with Princess Zelda/any other female protagonist from the game of your choice? Finn and Jake encountering the triforce and getting ULTIMATE POWER, only to use it doing something silly? I think there are a lot of possibilities there!
And I'm honored that you actually want to do fanart! I wouldn't expect you to until I sent you a whole set of the books, though.