I will be shocked if one of the guest characters is not a DC character. We had an MK character in Injustice. Why wouldn't we have an Injustice character in MK? Particularly since MK has already had guests like Freddy and Kratos. That's a discussion I'd like to see: which DC character do we think would be likely to be a guest in MKX.
Companies cross-promote. That's just what they do. Freddy was a guest in MK because WB had the Nightmare on Elm Street reboot coming out. Scorpion was a guest in Injustice because NRS/DC/WB are under the same umbrella. It would be amazing if there wasn't a DC Injustice character in MKX, particularly if they are planning an Injustice 2. I'm not saying both guest characters would be DC, but I would bet that one is.
My best guess is on The Joker.
1) He is iconic. Everyone knows The Joker. If it is DLC you want someone everyone knows. Scorpion is iconic, perhaps the most iconic MK character, so he went into Injustice. You want DLC that will sell or what is the point. Casuals, even those that didn't play Injustice, would still get Joker. This is going to knock out a lot of the cast of IGAU, since you want a must have that everyone knows.
2) He's died a lot. Joker has died in movies, died in comics, and died in video games already. It wouldn't tear the fabric of reality to watch him get a Fatality. This is going to knock out a lot of the IGAU cast, especially heroes, because there are plenty that we'd never see getting a fatality. It is hard to see DC being okay with having Batman's face melted off or Superman's spine torn out. Most of the hero cast is gone on this basis alone. Maybe 30 years ago it'd be weird to see Joker have his arms ripped off, but it wouldn't seem like a big deal now.
3) Joker is in the Suicide Squad movie coming 2016, brought to you by WB and DC: BOOM, more cross-promoting.
I suppose it could be Grundy, since people like zombies, even if he isn't as iconic. Maybe Bane since he is pretty well-known. There are some other contenders, but I'm guessing The Joker is a guest character in MKX. I could be wrong, but it makes a lot of economic sense to me. A popular, iconic character that everyone knows, lots of casuals would want to buy, who would fit in the dark, violent MK world, and would help promote Injustice/Injustice 2 and an upcoming movie.