There are so many characters in Smash Bros. Ultimate that most people thought would never happen. Banjo and Kazooie made their way over, Joker dropped by, and even San from Undertale is represented through a Mii costume. Still, with all that said, there are other franchises that seem like they'd be downright impossible to have in Smash Bros. Ultimate. Mortal Kombat would definitely be one of those.
While having someone like Scorpion or Sub-Zero in Smash Bros. Ultimate would be next-level surprising, that doesn't mean people don't want to see it happen. Ed Boon, co-creator of Mortal Kombat, is definitely on-board with the idea. When Shacknews asked about Boon's thoughts on Mortal Kombat getting recognized in the Smash Bros. universe, here's what he had to say.
From my perspective, me personally, I can't speak for all of Warner Bros. or anything, but it would be a like a stamp of approval. Like being part of the gang. I would love to see something like that. ...that would be a really cool thing to see.