After hours of research on Tumblr posts/threads, I’ve made a pretty good theory wrap-up to the events in the show, and what questions have to be answered. Here we go…
One day some military thing was dropping atomic bombs that would turn out to destroy the world. Thousands of years later and the appearance of magic and we have Ooo. The main conflict in our series, however, was that the Lich appeared perhaps around this time. Some speculate that he is a radiated man, or a product of the birth of Marceline(explained in detail later), or that he entered through a universe rip into the world from the sheer force and power of the bombs. In an attempt to save the world, Simon Petrikov unleashed the full power of the crown(and here turned into the Ice King) and froze the entire world, letting people mutate until they thawed. When they did, Billy(a heavily radiated man) stopped the Lich and imprisoned him inside of the Candy Kingdom. Around this time, Billy found the Enchiridion and put it in that fortress(where Finn had to go through the trials in that one episode), and gave the gems to several princesses, and to mix it up, the Ice King.
A large problem in the series is Marceline, with her strange past and aging. My theory is that she was always half demon and was sent to the world about the time the Lich appeared(coincidence?)and aged at a slower pace, combined with minor powers. Either her father bit her to give her more powers to survive, or a vampire bit her that came from either radiation, or through the same rip the Lich came through, if you believe that theory.
BTW, Finn saw the Lich in Jake in his wish universe because that was either a minor connection to the real world, or because the Lich HAS to exist, and Finn;s wish just prolonged the amount of time before it appeared.
Finally, some questions to look out for:
Who is Marceline’s mother?
Who was dropping bombs and why?
Where will the Lich go?
Who is Peppermint Butler? Who is BMO?
Who is Susan Strong?
Who are Finn’s parents? Who is his sibling? Did they survive?
How did those who survived survive?
I appreciate all of those who read this, and I would love for you guys to give me some additions, give rebuttals, and ask questions. Again, thanks for reading, and stay tuned for more! :)