Braniac-5I am busted: I know he's a formidable exemplum of genius in science fiction DC, but I've read a fraction as much about him as the other folks I mention, save for Inspe---you'll see. But what I know about Legion of Super Heroes means it's always at least in the 30th century. Rick doesn't do time travel. There's your loop hole. So, that's it for that. On the other hand, in the pilot, Rick DID go to "a future dimension" to get Morty some broken leg serum, though maybe he avoids that so he doesn't have too much fun and lose his portal gun charge. Parallel dimensions, hanging out with the Flash...there's ways, but, you know, good thing Rick and Morty isn't a story engine concerned with making a plan match up like so, because time travel, once it is out of the bag, is either a broken mess or a considerable part of your storyline as you keep your details all moving along in this strange order. He's more likely to be eluding Green Lantern than working with him. Fighting evil alongside the Flash and his friends? Yeah, give me a break. He doesn't have enough black in his union suit, which is much more in the Albert Einstein “I could care” or Bruce Banner “I am going to rip it to shreds probably anyway” mode, lab coat short hand and brown pants handy for handling accidental loads when you meet things you didn't expect and they seem hungry. Let's face it, if he'd showed up in a Lee/Kirby joint around 1967, Rick Sanchez would've turned out to be the super-villain! To give the story heart, Morty would win the FF over, probably with help from Sue Richards, and I could even see the duo and the quartet parting on good terms, albeit mixed reactions.
Hey, what if Rick found the guy who disappears to die nobly in the Negative Zone in “This Man, This Monster!” from FF#51? Yes, we geekazoids love our shorthand. And no, let's leave at least somebody at Marvel Comics dead.
Finally, let's see Rick and Morty meet Inspector Gadget!I remember when that was honestly the very best super hero cartoon on TV. How about it, Uncle Gadget? “I'm always on duty!” Penny and Summer could carry an episode thread ably between them. Imagine: lines by female characters that aren't about the male characters. I mean, really unusual, creative stuff!