Well, it's possible for them to do that, but to do that, and then not actually include those characters in the VS. mode would be kind of wasteful.
I mean, I already thought that about Raiden, Quan Chi, etc. All the guys in MKD, who you get to fight in Conquest mode, but aren't on the roster. I would've loved to play as Quan Chi in MKD.

But now that I think about it, constant battles would probably work for the story mode. It worked for Def Jam games. And although I really don't think I have anything to worry about, I'm just hoping that MK vs. DC isn't gonna barrage you with text messages on your in-game cellphone, between every battle.

It's not a very admirable way of doing business, but lord knows we've been seeing it since the first MK! And let's face it, MK fans haven't historically been all that picky about what kind of arbitrary move sharing goes on!

By throwing skins over the Superman build alone they could get versions of a lot of NPCs without having to build new movesets (ala; Bizarro, Supergirl, Powergirl, Martian Manhunter...) and by adding and subtracting one or two moves, could go even further.
And as if to come full circle; you've got to imagine there'd be plenty of fans who'd actually like seeing Scorpion in a rainbow of alternate colours (and nametags).

I don't know about probable, but it's definitely within the realm of possibility.