Mortal Kombat x Street Fighter is something fans of both series' have been dreaming of for decades now, yet its never happened. We do know that Mortal Kombat co-creator Ed Boon tried to get the ball rolling on a collab, but it never came to be. In a recent interview with Eurogamer, Street Fighter producer Yoshinori Ono went into more detail on why the idea was shot down.
It's true that a proposal for a Street Fighter character in Mortal Kombat was rejected by Capcom. But it wasn't me personally! There were many people at the company that felt that it wasn't a good fit for our characters. I actually met Ed at the Brazil game show and spoke to him personally about it. So it's true - but I didn't make the decision! (the two worlds don't mix) was the major concern of our licensing.
We don't necessarily rule anything out. We've had our share of crossovers over the years - Marvel vs. Capcom, Tatsunoko vs. Capcom. From Ed Boon's side they had Injustice that had various universes colliding. It didn't work out this time, but Capcom is interested in doing that kind of thing. If we can get something we feel good about, we could move quite quickly to make that happen.