What I want for DLC, first and foremost is well-tested, well-balanced code and content. Don't rush anything, take your time and do it right. Don't just throw in a half-baked new character/stage/kostume/whatever in there just for the sake of having a new character/stage/kostume/whatever. If it's a new character make him/her balanced compared to the rest of the characters, with unique look (meaning his/her own 3D model) and moves of his/her own. I don't want to see a palette swap of Scorpion/Kitana/whoever with a moveset that consists of half of Sub-Zero's moves and half of Smoke's moves (for example). I know the temptation is going to be hard now not to push out as much content out there while the game is hot to make as much money as possible, but remember NRS, Quantity over Quality is always a win in the long run.
With all that said, what I want to see as DLC is Reiko. He was one of the best new characters in MK4 and had a cool moveset.
Update: or Frost. I liked her a lot in MKDA.