In part, all of the comics I've written are my take on what could have happened after the episodes. They're still technically my ideas, though working within storyline. The "working within storyline" bit is the fun and challenging part. It's so easy to go wild after all.
Though I never mentioned an endgame before, if things proceed as normal (as "normal" as Adventure Time can be) with no radical change, I do have an endgame for Marceline and Simon Petrikov, which I've written in "Adventure Time -epilogue-" a few months ago.
Well, when Finn wore the Purple Gem from "Daddy's Little Monster", he turned into a Monster right? Let's just say that being tainted by evil leaves something behind that can only be seen by someone who's been there.