Time travel, Cross-dimensional and crossover idea:
Okay, let's take a step back first to Samurai Jack. He was locked in vicious combat with Aku the shape shifting demon. A creature that once upon a time was some sort of blackish slime before taking the form we're more familiar with. Jack does eventually defeat Aku and is able to return to his home time. In doing so this alters the world's timeline, such that the world and time that Jack had his adventures in never existed because the events that led to it were changed.
The nudge that prevented Aku from rising resulted in the black substance of his body laying undisturbed until the modern day. Cue scientists getting their hands on the material and realizing it's some sort of mutagen. The black stuff forms the basis for the mutagenic bombs that get unleashed later on during the Mushroom War and leading to the land of Ooo. In this scenario, it is the Lich that is created rather than Aku. It is still the same essential essence of EVIL just now in a different form and calling on someone else to rise up against it. The Lich ended up imprisoned in Amber (tree related once again. Something perhaps about the natural world being inimical to the Evil that is Aku/the Lich), but once free, someone needed to be called up to deal with him.
In canon, that's Finn. But what if Jack gets called back to deal with his old nemesis.
There's also the fun possibility of Jack acting as a sort of mentor figure for Finn. Not to mention the possibilities of Jack just discovering the new world around him which was created by the defeat of Aku. It is still a strange world, but one which is much brighter than the one Aku ruled over.
Granted this is still just very, very rough, but there might be something here.