Here are some theories to discuss, they seem pretty legit to me except the fifth theory, and part of the third.
1. Finn's Found/Birthday. We can see in "Mystery Train", that it is Finn's birthday, but if we look at the airdate, that's March the 14th, now, don't go all "So Motor, that's just airdate" and whazzows like that. It's 3/14/???? 3/14 = 3.14... π (Pi), and Finn used to say "Mathematical!". Geddit?
2. The Possible Mushroom War Date?
On the back of the Enchiridion, there are three numbers: 8, 13, and 21, August 13th, 2021. You may say this is the Fibonacci Sequence... But what day of the week with this be? Friday the 13th.
3. Origin of Raincorns.
The Mushroom War is meant to be maybe World War III. But right, the North Koreans are being stupid. Now if we think about it, Rainicorns speak Korean and like the taste of Humans, what does this mean? It may mean that the North Koreans wanted to create a flying animal that could blind their enemies with colour, so they tweaked the DNA of horses and trained the new Rainicorns to kill and eat other humans, but after the nuclear fallout, the Rainicorns broke free and ate all of the North Koreans.
4. Susan Strong is Finn's mother.
Well, in the alternate timeline, Finn's mother has blonde hair, and Susan has blonde hair. Also, when Susan first sees the sun, she is afraid of it, but Finn comforts her by tell her it is the sun, Susan then looks at him and says: "Sun?" or did she say: "Son?"
She has a similar body build to adult Finn too.
5. Susan Strong is Fionna, and Prince Gumball is Princess Bubblegum's Uncle.
WARNING: THIS ONE MAKES ME LOOK LIKE A TOTAL FOOL EVEN THOUGH I DON'T AGREE WITH IT, I JUST WANTED TO SEE SOME FIONNA FANS' REACTIONS.
Fionna has blonde hair, Susan has blonde hair too, maybe, Fionna found the Hyoomans when they were humans, so they decided to start an underground colony. Fionna went with Cake and Lord Monochromicorn, but when they went down there, but something down there mutated the Humans into fish people, but with Cake and Lord, they merged together and created the Lub Glubs, and that's why the Lub Glubs can stretch and are shadow-like (Cake and Lord start reproducing as the creature and that's why there are so many). Fionna managed to escape but lost her hat (you can see in the title card in the "I" that there is a woman wearing Fionna's hat, and you can see a hat like Fionna's hat in the debris in Beautopia). Decades later, Prince Gumball went to look for Fionna, Cake and Lord in the Candy Cotton Forest, but couldn't find it, so he chooped all the trees down, but still couldn't find it in the end, that's why PB mentions her uncle, but gets nervous when Finn mentions he gets weird when talking about Humans. Overtime, Fionna forgets her past and her language, but remember's Cake, so she makes a hat in memory of the cat, but thinks Cake is purplish-black, due to Cake's transformation.
Prince Gumball also becomes single, that's why his throne is handed down to PB.
So discuss! What do you think?