My guess is either Injustice 2 or Ubisoft's new IP. Injustice 2 because it just isn't as big as Mortal Kombat, and wouldn't really gather a lot of attention if revealed during E3, this is also assuming it couldn't get into Microsoft's or Sony's conference, more likely Sony's since they usually show off WB games, and with SFV and KOFXIV, a multiplat fighting game might not be a priority for their marketing. One thing against this is I don't think they'd have a whole month of coverage for IGN First this early, besides new fighters and story info, but I figure they'd want to spread a lot of that news out slowly.
I think Ubisoft is more likely, since we do know they'll be revealing their new game this week, which makes me think it'll be revealed by either IGN or Game Informer, as the only reason it makes sense to reveal it outside of the conference, a week before, is if they're planning to have a whole month of content for it.
It could be Rockstar, but I don't really think they need to rely on the coverage, but still a possibility, and the third most likely. 4th guess is Forza Horizon 3, since that's not a huge loss for their E3 conference.