To quote Frieza from the Team Four Star dub of Dragon Ball Z: "Well that happened." Of course I am referring to the royal mindfuck that was last night's episode of Adventure Time, "The Lich." Now, even as I am writing this, people are speculating as to what will happen next season. Here are my theories:
1a. The Finn and Jake we see in the finale are an alternate timeline where one key event is different, namely the Mushroom War never happened. Every other event remains the same. In this theory, Marceline is still a vampire because her becoming a vampire is independent of the Mushroom War and Simon Petrikov becoming the Ice King still happens as him discovering the crown is independent of the war. However, the two characters never met because their meeting is the result of the war. As a result of the Ice King's "wizard eyes" he can see what happened and restores their memories of Ooo. They go on a quest to find the gems and the Enchirideon of that dimension with the help of the Ice King so they can return to their own timeline and fight the Lich.
1b. The Finn and Jake we see in the finale are an alternate timeline where one key event is different, namely the Mushroom War never happened and the Enchairideon and gems were never created. Thus, Simon would not be the Ice King and Marceline would not be the Vampire Queen. BUT considering that Finn and Jake lived over a thousand years after Simon and Marceline, both would be long dead however their descendants would be alive. With the help of Simon's descendants, Finn and Jake find a way to return.
1c. The Finn and Jake we see in the finale are an alternate timeline where one key event is different, namely the Mushroom War never happened. The Finn and Jake from the Ooo timeline will appear and team up with the alt-Finn/Jake and the Ice King and Marceline to get back to Ooo using that dimension's Enchirideon.
2. The Finn and Jake we see in the finale ARE the Finn and Jake from Ooo Prime but altered to fit into the new dimension like how we saw Stewie and Brian in "Road to the Multiverse." It is a covert operation to track down the Lich culminating in a showdown using the universes with Finn and Jake and their gender-swapped versions vs. the Lich.
Personally, I think none of these will happen as they are too easy to figure out. It will probably be some new outrageous mindfuck that will leave with more questions than answers. Penn, meet J.J.