Street Fighter and Mortal Kombat have been at the forefront of the fighting game scene for the past three decades. In all that time there has never been a crossover between the two juggernauts, and Ed Boon's admission that he wants a notable Street Fighter character in a future Mortal Kombat game suggests it's not for a lack of trying.
The Mortal Kombat director has always been very open on Twitter when it comes to answering questions about the series and other NetherRealm games. When asked who his dream crossover character would be in a Mortal Kombat game (thanks, ComicBook) Boon replied Ryu. The gi-clad Street Fighter character who has been the face of the series since its inception.
As for the likelihood of that ever happening, while you'd have thought it would have been a thing by now if it was on the cards, you can never say never in video games. Kratos and Master Chief and both in the same game thanks to Fortnite, along with a few Street Fighter characters, so it certainly seems feasible that Ryu might do battle with the likes of Sub Zero and Johnny Cage someday in the future.
Boon has previously revealed the team at NetherRealm votes on what they want the next Mortal Kombat crossover to be, leading to the likes of The Joker and RoboCop being in previous installments in the series. It's not as simple as deciding on a name and adding them to the roster though, of course. NetherRealm will then have to strike up a deal with whoever owns the IP, and it seems a deal between it and Capcom for Ryu's rights has never been reached.
Even though they have led the way in the fighting game world since the early '90s, Street Fighter and Mortal Kombat are very different. While both violent by nature, Capcom might not like the idea of Ryu jumping ship so he can rip someone's throat out, or have a Mortal Kombat character quite literally roundhouse kick a hole in his chest. The studio will be focused on Street Fighter 6 right now which launches in June 2023.