Can you imagine how amazing this mashup would be? Humanity has one last hope to combat the Titans: Jaegars! Imagine this: Mako and Mikasa mind meld and end this pesky Titan problem, probably by sticking a bomb in a giant ocean abyss. AWESOME.
This mashup makes so much sense. Both stories feature a big cast of complex characters with involved back stories whom we care about even though they're all bad people. We have fun following them as they pursue "justice" or "power" and destroy the world in the process.
The Land of Ooo is a post-apocalyptic wasteland filled with mutants, so it ain't hard to picture Finn getting into trouble in the American commonwealths. And Jake would definitely have an awkward encounter with a traditional, normative dog.