a) Ever since I heard about how MK2's Kintaro evolved from an original concept that basically boiled down to being a sort of Tiger-man/Weretiger fighter (who was originally going to be a playable character instead of a boss), I always wondered what they could have done if they actually followed through with that idea. Werewolves are seen fairly often in fighting games, but there aren't too many Werecats of some sort. Could end up being very interesting, especially if he and Reptile ended up as rivals (playing up the old "Tiger" and "Dragon" rivalry in Asian mythos).
b) For a series that's all about maiming and murdering people, we've never seen Boon and them do more to involve some one who's actually more along the lines of a normal looking, psychopathic serial killer or whatever. Hell, that much ended up being why I liked the "MKRebirth" thing when I saw it last summer. Give some one relatively unassuming, like an "Ichi the Killer" type, some type of interesting super power/martial art or otherwise, and let's see what happens, shall we? If there's one thing I didn't like about MK going for the super-natural wuxia theme by MK2, it's that it did a lot to down play the notion of more "normal people" like Stryker having a reason to be up in the mix. And I could stand to see that changed.
c) By later in the series, the Red Dragon supposedly were experimenting with Reptile and one of those two dragon's DNA to make their own "super dragon soldier" hybrids amongst their ranks. Well, what's stopping NRS to put that ahead of schedule and introducing some one like that sooner rather than later? Could be either a willing participant, or the possible "They dun taken away my HUMANITY~!" type of character. In any case, could be interesting, or even a means of re-vamping a character like Chameleon or Khameleon.
and and perhaps an alien like character