Freddy was in because Warner Bros (who owns NRS and DC) was rebooting the Nightmare series around the time that MK9 came out. WB likes to cross-promote, as do all successful entertainment companies. Thus, it was no surprise to me at all that Scorpion made it into Injustice. This cross-promotion thing is a reality that NRS fans should just accept. The cries for a "pure" DC fighting game fell on deaf ears, though the prospect of Scorpion winning EVO definitely got a reaction!
Following this logic, my prediction is still to expect an IGAU character as DLC for MK10. I'm guessing Joker. He is iconic and fits within the dark, bloody world of MK, at least the Killing Joke style Joker does.
