{So this theory is various so bear with me.}

Finn is in a coma, Finn is imagining everything, or Finn is role playing with his friends.

1)  OK we all know Adventure Time is defiantly beyond reality, but some people wonder why it is so.  In fact the belief that he is the only human, and that there was a war 1000 years ago brings up further questions on the matter at hand. This theory states that Finn was knocked into a coma when war broke out. At the time he was an ordinary kid with his older step brother Jake, his video game system BMO, and his friends from school. His dad abandoned him when he was a baby after his mom died. He was later found by Joshua and Margaret ( who were not dogs, just normal adult detectives ) and the couple took care of him until they died as well. At the foster home they met new kids and went on adventures until Jake was old enough to take care of Finn with his girlfriend. About when Finn turned 12 a bomb hit their city as the war begun. Finn was knocked into a coma and had imaginary adventures while eternally asleep. 

2) So this one is similar to #1 in the likes that he's imagining his life, but in this one he's more of just imaginative. Finn is a 13 year old boy who was forced into a bunker to wait out the war. Extremely bored he created an imaginary world in which he could do whatever he wanted and be a hero. In the bunker with him he exaggerated the appearance of almost everything and made up alter egos for them. Jake his dog became a magical talking dog brother, his hand held blue gaming device became his living consul that could talk, cook and do karate. The girl who always chewed gum in the bunker became Princess Bubble Gum of the Candy Kingdom, and since she brought down lots of dolls he made them her candy citizens. Another noticeable girl in the room was Marceline. She was always having fun and scaring kids, but after cheered them up with her music and base guitar. Since she looked demonic he made her Marceline the Vampire Queen. The list goes on for these characters so I am going to end it here. 

3) This theory is my favorite of the three. Finn, Jake, Bonnibel, Marcilene, and Beemo are all high schoolers who hang out at lunch and after school. They decided to start a role playing club with some of their other friends and kept adding on to the original story with more side characters and more adventure. They enjoyed making the characters abstract and funny to go with a goofy and free theme and called it Mind Adventures until changing it to Adventure Time later.

OK so you may have noticed that I took the main ideas of the theories and made my own stories with them, and sorry if I didn't please you with the story. Anyways thanks for reading and if you want to add on or tell me something I missed please comment below.