Well, I DO see what you mean by "not being AT anymore"--is it me or does it seem as if Fionna and Cake are a bit too much more...how should I say this..."mature" that the OC?
In fact, I dunno about the rest of you, but, for me, it seems EASIER to make a romance fanfic about the genderbent world than the original AT world. Finn just doesn't seem to be meant for all that romance crap--but Fionna definitely does.
...Anyway, I love the genderbent Adventure Time universe. Better than the original one, I won't deny that. But I only like the genderbent universe better because it's way better you want to make a mature take on an Adventure Time fanfic. Of course, you can never replace Finn and Jake--it's better to use the original AT universe when you want to make fanfics that...aren't so serious or mature.
...Just stating my opinion. :D
...Oh, yeah, and I forgot--I have this..."theory" as to why it seems the Fionna and Cake episode wasn't very...Adventure time-ish:
Technically, the story of the Fionna and Cake ep is the Ice King's fanfiction. And because of that, the story and the chatacters are set in the world and attitude the way Ice king sees it. I guess that would be the reason it didn't feel so AT-ish--if the REAL writers of the episode made it very much like Adventure Time, then it just wouldn't make any sense being Ice king's fanfic, right?