Hey everyone! I've been very interested in the Mushroom War and all those things of the past, probably like most of you. So I have made my own theory about everything.
Before the Mushroom War, Simon found the crown which began to turn him into the Ice King and made him lose his fiancee Betty. But what if Simon also had a brother, Finn's father. (I call him Fabian). "Fabian" married with Inngrid before the Mushroom War and also followed his brother's idea to travel the world. Before he went on his journey he got the Enchridion from Simon which may help him in his journey, he said he wanted to donate it to a museum but decided to give it to Fabian.
Along the way Inngrid got pregnant. Time passed and the Great Mushroom War began. During the war Finn was born, much has changed and the Lich was trying to rule Earth.
Fabian could make a phone call with his brother who sounded insane. Simon said their parents had died and Betty. Simon began saying crazy things and hung up. Fabian and Inngrid, along with Finn, saw it as their task to defeat the Lich. Before they went to fight the Lich they encountered the Cosmic Owl who took Finn with him to keep the human race save. Fabian and Inngrid locked the Lich up using the Enchiridion but also took much damage themselves, in a search for their son they eventually died.
The Cosmic Owl had frozen Finn for 1000 years, when that time would've passed Finn would melt and would be raised by Joshua and Margaret. After Fabian and Inngrid died they still layed there in the forest. Simon, who now was the Ice King took and froze the body's. The bag of Fabian contained his sword (Golden Sword), the Enchridion and the Flute. Inngrid had a hoodie that was also found. Ice King took those with him too, but it is unknown how the lost the items again and how they ended up on different locations.
So what I actually try to say is that Finn is the nephew of Ice King. What do you guys think of this theory, it would be a cool plot twist. But maybe some of you may think it's lame but I still find it better than theory's saying that Finn is in a coma or that Fionna is the older sister of Finn.