Sure, any crossover would spark interest but this one comes down to one aspect for me and it isn't violence. Gameplay. Nintendo takes it extremely serious while the guys over at Neatherealm care more about graphics.
I was once a strong MK supporter. I grew up playing UMK3 and in the arcades. Trilogy, although ran on the same engine, was the mark of that franchises downfall.
I was just saying how MK is easily one of the worst examples to make the 2D to 3D transition. Mario? Arguably the best.
Now if a kombatant made his or her way into Smash, the whole nature changes. Nintendo, imo, would most likely show NR how it's done with smooth animation that actually makes sense.
MK needs to reinvent itself in a more fluid and polished animate before even considering making guest appearances.
Especially in a game produced by the masters.

Insert Coin. Press START. You Died. Continue?