How about this?
We know from the episode coming up that The Ice King crown was supposed to fight off a comet to protect Dinosaurs and stuff, so with this second comet, maybe Ice King's basic goal will finally be met somehow to stop this next one, and it stops going, essentially shutting down the Ice King (protip: not really, just hang on I'm going somewhere with this) So once again, the Ice Kingdom melts, and his land is up for grabs, situated between the Candy and Fire Kingdoms.
Finn is trying to decide how to get this addressed peacefully, and it's when Simon's will is revealed, we see he left his 'home' to Marceline the Vampire Queen. So Finn's like 'dude you can end all of this' and Marceline's like 'I don't want to' so Finn goes Goblin Kingdom and basically fights her for the control of her royal titles and lands. It's just to get her to agree, but they go at it pretty crazy, and then, suddenly, Finn wins. Now Finn is the successor of the Ice Kingdom, the Vampire King, and the owner of the lands inbetween, and has basically set everything in his court. Instead of fighting and badgering Finn, now they're seeking a partnership with Finn. Essentially, Finn can now marry Princess Bubblegum or Flame Princess. They're visibly uncomfortable, but royal marriage, you know what's up. Of course, now that Finn has his girls at his beck and call, it's gone all Monkey's paw. He chooses one girl, the other tries to attack him, and now Marceline won't talk to him either. Even Jake has to stay wtih Rainicorn.
Now Finn is stuck trying to undo this complex romantic and succession plot to just get all his friends back to liking each other, and liking him, and just wanting to adventure. It's sort of a microcosm of what's gone on, the plot and drama has become so tangled its choked Finn out of doing what he likes to do. In a way, getting out of this mess is itself an Adventure. So, there's still Betty and Prismo or hell, left field, the comet is a new princess.