Introduction
Thank you everyone for your feedback and comments regarding the information I have presented so far. I had no idea there was so much information I missed. Because of that, I have updated this article with a new segment where I address the new information I have received.
Fionna + Cake = Susan Strong?
In my previous chapter, I said that Fionna and Susan Strong have strikingly similar appearances and are likely both human, so Fionna must be Susan and her hat must be Cake. However, several people brought to my attention that in the episode "Beautopia," a Lub-Glub is seen chewing on a skull with a hat that is similar to Fionna's, concluding that it must have been Fionna's skull. While this quite possibly disproves my original theory as to what happened to Fionna, it just might prove my overall theory that Fionna and Cake lived in the same world as Finn and Jake, in the past. Because of this, I have come up with a couple new theories:
One possibility is that Fionna and Cake stumbled upon Beautopia when it was overtaken by the Lub-Glubs. They had no fire with them and were unaware that that was what defeated them, thus the creatures were too strong for them and they met their untimely doom. They probably didn't have any connection to Susan Strong, and her back-story remains a mystery.
Another possibility is that Fionna made Cake into a hat and ditched her old one, which prompted a Hyoomen to wear it. This Hyoomen was attacked by Lub-Glubs when they took over Beautopia while Fionna grew into Susan Strong and saved as many of her new people as she could. This one is a bit of a stretch, but it just fascinates me how similar Susan and Fionna are.
PB and PG are related?
I am told that at the beginning of the episode "Susan Strong," the uncle Princess Bubblegum mentions is named Gumbald, not Gumball. I went back and watched the episode, and it does sort of sound more like Gumbald. This sounds like a strange name though, so I'm wondering if it's a nickname, or if he was Prince Gumball's brother or something and Bubblegum was Gumball's daughter. I know you never actually see a little Bubblegum in the gender swapped episode, you could make the argument that she was only that: never seen. In the regular episodes, Bubblegum doesn't seem to have any other visible family wandering the castle. Not even her parents are around (but I won't give you the old "there's no king or queen" rant). There's probably not enough information for me to construct an entire theory, but I still think it's possible that the two are related simply because they seem to be the only human-gum hybrids.
The Rulers of Ice
There's not much to say about this section other than an apparent announcement by the genderbent characters' creator Natasha Allegri that the Ice Queen has a completely different backstory from the Ice King and got her magic tiara a different way. While this does alter my original beliefs a little bit, it's still not specific enough for me to really change how I initially perceived the backstories of these two.
Closing
As always, comment if I missed something. The more information I can get, the more I can build upon these theories and come to a conclusion about what happened to these characters.