This is an early theory before recent episodes
Steven Universe is a Cartoon Network show that deals with aliens and fighting for the earth.
This theory is one that connects SU and Adventure time.
So if you keep up with Steven Universe you would know that at the moment the gems are battling a giant gem or cluster that will destroy the earth. These two shows may seem to not have anything in common, but what if Adventure time was only a theory. Lets say the Crystal Gems do stop the cluster but in doing so they get destroyed, what would happen?
well without the gems there to protect the world from corrupt gems, there would be complete turmoil and who knows maybe even a war. the Great Mushroom war. the great mushroom war is what ended modern life as we know it and was sort of the apocalypse, it was basically a nuclear war. Anyway after 1,000 years, by Marcy's standards, Adventure time takes place...
And though this theory could be unlikely there are several real world links. For Instance, Rebecca Sugar, the creator of Steven Universe worked on Adventure time and wrote most of the songs for it. And yes I do realize that SU comes out after Adventure time, but that doesn't mean that Rebecca Sugar didn't have the cluster killing the gems in mind.
Any way like I said this is just a theory...
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Adventure time Theories
FanfictionThese are some of my theories about adventure time enjoy and if there's something specific you want me to write about comment I will come up with something ;p