I'd like to see Nightcrawler and The Thing hang out sometime. They seem like they'd get along well. Both affable, friendly guys. The exterior can seem fearsome, but they're actually softies. Best friends with a couple of hotheads*. Both enjoy beer. Kurt fancies himself a swashbuckler. Ben was Blackbeard the pirate.
I don't think they've interacted much. They were both in Secret Wars, but the X-Men split off fairly quickly. Kurt was out of commission during the Fantastic Four vs. the X-Men mini-series. Ben's off in space right now, but that's fine. Kurt's done the outer space thing before. And Kitty's out there now, too, so he could visit her. Although I'm leery about adding another character from a different franchise to the Guardians of the Galaxy, but it could just be a short stint.
They could use his help with something - maybe they need a teleporter that's not technologically based, or they could just use a wild card - and Kurt's not the sort to turn down a friend. Then there's time for sitting around and talking, or getting into mischief. Kurt and Ben get into a high-stakes intergalactic poker tournament against the Grandmaster, or they help some rebel group overthrow a brutal dictatorship (that just happens to be run by Star-Lord's jerkwad dad, probably).
* I admit I'm counting Johnny as Ben's best friend, rather than Reed. I could see Ben still being Reed's best friend, but I feel like Ben has other best friends. But it's hard for me to picture what being best friends with Reed would be like. Doesn't seem like someone to whom hanging out would come naturally.