At first, I thought no, but then, thinking about it, I think it could work. They'd have to have the Smash MK character draw out sweat instead of blood, in an homage to the original SNES controversy. Sub-Zero would be the obvious choice since he's iconic enough and could have a fatality where he just froze people and shattered them, a fitting fatality to represent the series while not involving blood and gore. For the Nintendo character in MK, they couldn't do any of the characters that were even remotely cute, good, or wholesome, but some villains could work. Even Bowser would be too wholesome for MK though, and I don't even think Nintendo would be fine with Ganondorf getting his spine ripped out. Ridley, on the other hand? Just imagine his skewering tail from the Smash reveal trailer on Mario, except uncensored, and against an MK character. Really, Ridley's the only Nintendo character brutal enough that I could see him fighting in MK, and since he's a monster no one would care if he were chopped to bits or gored. He'd have alien blood too probably, so Nintendo could even avoid that issue. He's had an exposed heart in the Prime games, and it was orange, so case in point, he's a fairly brutal character and a good fit for the series as far as Nintendo characters go, while also having likely orange blood so Nintendo can avoid any more gorey an association with the character than necessary. It could be a way for Nintendo to court a hardcore audience they don't normally reach to try out a critically acclaimed but underselling franchise that might appeal to them. Prime 4 deserves all the spotlight it can get.