Guest characters are as far as they need to go crossover-wise

MKDC2 will probably happen and we all know it. 3 million units sold says so.

as far as hypotheticals: Mortal Kombat Vs Street Fighter.

The only crossover will be Mortal vs Kombat for all I care.But seriously, I think that the developers got already what they wanted from MK/DC (fanbase, money) so there's really no need for another crossover anytime soon - especially when they are about to kick start the whole series again.

Mk crossover equals watered down Mk game :(

This.
Although, if it HAD to be something, Killer Instinct would be my choice.

If they want to make another crossover with DC, I'd say they should focus less on superheroes and stick with villains. Batman villains seem to be the most popular and dark from the bunch... so something like Mortal Kombat vs Arkham sounds good to me. Joker, Scarecrow, Killer Croc, Bane, Clayface, Mr.Freeze... even Catwoman and Batman - they'd fit the grim and violent tone of MK a lot better than Captain Marvel, Wonder Woman or the Flash.

I would like to see Mortal Kombat crossed over with Mortal Kombat that way I can just play Mortal Kombat without having to worry about non Mortal Kombat characters getting in the way of Mortal Kombat.

Did you get it that I just want to play MORTAL KOMBAT?