Mortal Kombat’s one of my favorite series too, but as of the last game I feel it’s had 3rd party guests, when it goes as far as them being there at the expense of characters from earlier games fans wanted back but didn’t get, though guests make most of the DLC money, so they won’t be going anywhere...
Ignoring that MK already had Leatherface, I’d prefer Ash from Evil Dead as a chainsaw + shotgun user there, even though Doomguy could make that kind of moveset work too, but he would be more appreciated in Smash amongst other gaming all-stars than he would at being hyped by MK fans. There’s no denying that much.
That reminds me, I did fancy the idea of Scorpion (with maybe Sub-Zero as an echo) in Smash as a Western 3rd party, but he’s got no real shot like Doomguy due to modern MK’s lack of presence in Japan and such.