Yeah, there's always that desire to glom your favorite things together and make a ridiculous Steven Universe and Rick and Morty crossover, but it's probably a bad idea. Gravity Falls would probably make for a reasonable combo, though I'm inclined to prefer that Steven Universe just not crossover. Beyond the show's tone, it has such a self contained little world, and bringing outsiders to it just feels... wrong somehow. It's no coincidence that the characters rarely leave the town except to go somewhere that has a warp pad. Even something as apparently innocuous as Gravity Falls would change things in these weird little ways that I don't think make sense for the show.