
Well it will never happen. Ed Boon offered Capcom and also the creator of Street Fighter Yoshinori Ono multiple times to make Mortal Kombat vs Street fighter happen but they declined.
Street Fighter is PG fighting game made to appeal to teens and kids.

Mortal Kombat is an M-rated adult fighting game with lots of over the top gore, blood and violence.

Capcom don't want their child friendly characters like Ryu or Chun-li getting dismembered and get killed brutally or kill vice versa.
Also many Street Fighter characters are underage because in Japan, underage superheroes are trendy because that's what kids like.

Child deaths can get a game banned in the west. All Mortal Kombat characters are adults for a reason.
Since Ed Boon can't get a Mortal Kombat vs Street fighter game, He tried to get at least some street fighter villains as guest characters for Mortal Kombat. Yoshinori Ono is open to the idea but the Capcom board said no, so He settled for Arnold Schwarzenegger as Terminator as guest character which sounds like a better option anyway.






