Although I like the Western crossover idea the most for now, I finished Attack on Titan the other day and couldn't stop thinking about an AoT/SSSS crossover. Mora is already walled off, we have main characters wishing to see the wonders of the outside world, and I'd love to see the crew flying around, slicing this comic's version of giants... Also, Lalli shares Mikasa's talkativeness, Tuuri has the cuteness and smarts of Armin, Sigrun is crazy enough for Zoe, uh, Emil and Jean share the same level of arrogance, and... I don't know, Mikkel would be a good coldblooded, sassy Levi, but he's not midget enough, and that leaves... Braidy... as... Eren? Or Potato girl?
What about a Fullmetal Alchemist one, since I have already trodden into anime land?