>>150812193
DUH see pic
>>150812328
The worst possible ending is them leaning excessively far into the josei thing they're going for, and make Fionna le unkillable mary sue girlboss who has ridiculously noticeable plot armor (but she cried so she's human and vulnerable!)

Then HW and Finn break up, and Fionna says something like
>Woah, radical! I'm bicurious! Wanna have a freaky sick threesome with your ex-BF?!
>Cake: YOU GO GIRL! Get that sex-positive energy flowing!

Obviously they won't depict that, HW will coldly shrug her off, but if they went full, disastrously, off the rails, I could see them trying to do it, leaning into the more adult subject matter.

Lastly, I think the awful cherry on the horrible ending cake would be them try and failing to pull off a No Way Home style plot, they have done a good job of establishing the multiverse in the Adventure Time world, but I can totally see them doing three straight episodes of Fionna saying
>WOAAAAH THESE ARE MY ALTERNATE UNIVERSE SELVES?!?!?!?! THAT'S SO RADICAL!

And having Fionna being ridiculously tumblr-written, overly commentating on everything, really it's a miracle they didn't do it this badly in the first series.