My Theory on the Mushroom War:
In the near future, all of the world goes into a humongous nuclear war, hence the name "Mushroom War."
In the episode "The Lich" an old Marceline talks about her old friend Simon, "The, (the ice king) who saved the world by blocking a large bomb with his body. This makes me wonder; could this alternate universe be the world if the great Mushroom War hadn't happened? In the episode "Simon and Marcy" There is a link to the Mushroom War. The aftermath of the world is presented with The Ice King, and a very young Marceline. As you may have noticed, this episode is almost the opposite of "The Lich" episode. There are Toxic Waste Monsters in this episode as well, which might be humans after radiation. Ice king was possibly immune to this, because of the crown which makes him inhuman. At the end of the episode, there is a pink substance which hands Ice King supplies for young Marceline. The substance then blinks with obvious eyes and mouth. In my opinion, this is Princess Bubblegum. Read more about that in my Princess Bubblegum Paragraph.
My Theory on Finn the Human:
Everybody who has seen Adventure Time knows about Finn, a fun-loving teenage boy. But what is Finn's backstory? Finn is the only human left on earth. But why wouldn't he end up being a "zombie" in my theory? Honestly, I haven't found anything that proves a specific theory about Finn, but if you do happen to find anything, I would love to know!
My Theory on Princess Bubblegum:
As I mentioned in my Mushroom War theory, The pink substance is what I believe to be a young Princess Bubblegum. As in the one of the most recent episodes called "The Vault" Finn has opened his "vault" of bad memories. He finds that in a past life, he was a young woman that had tried to steal something from the one, the only, PRINCESS BUBBLEGUM. In past episodes she has said she is nineteen, but both The Vault and Simon and Marcy prove that Princess Bubblegum is WAY older. At the end of The Vault, Finn finds the bones of his past self, and he also finds the Princess' stolen item. His past life also channels through him and tells Bubblegum that she is WAY OLDER than 19 and that she should stop lying. So, how old is Princess Bubblegum? One thousand years or more by my "calculations", but who knows?
My Theory on Ice King:
In the Adventure episode "Holly Jolly Secrets Part II" Finn and Jake watch secret recordings of the Ice King. In one of these recordings, they find the Ice King, but human. In these recordings, The Ice King documents his life after he finds a mysterious crown. This crown will end up driving him crazy, as seen in the episode Simon and Marcy. But strangely in the recordings, he always records near a window. At first his life is normal, he has a girlfriend named Betty, and the view out the window is quite normal. What happens next is very odd. The world is not being bombed, but is covered in snow, this leads me to believe two things.
The bombings actually did happen, and The Ice King froze the world with his crazy mind, to save himself. He also mentions that Betty had left him in one of the recordings, and most people went to believe that she had broke up with him. But what if she had ACTUALLY WENT? What if he froze the world because he wanted to stay with his dead lover forever?
The bombings actually never happened. The freezing of the world caused plants, animals, and basically the whole world to adapt to the cold environment. The world eventually thaws, but the ice king is so grief stricken about Betty, that he constantly tries to capture and rape women, trying to find the love he lost.
My Last Theory:
This has been mentioned a couple times, but what if the whole new world is literally just a figment of a boys imagination?
In this theory, a boy is terribly neglected and his only companion is his old dog, Jake. A few months later, Jake dies. Finn then tries to kill himself, but instead falls into a coma. In his coma, he adventures with Jake around a boys fantasy, with evil villains, beautiful princesses, and his best friend. Forever.