I wish there were more unlockable characters in MK, rather than just a boss or two. It doesn't feel like there's much of a reward for playing through the story on harder difficulties for example, even though the MK9 story mode was awesome.
I'd like it if only 8 - 12 characters were playable at the start (or if you wanted to really pay homage, the seven from MK1) and the rest were unlocked through playing various modes, and tied to achievements. I speak as a mainly SP/casual MP guy, though, so I'm not trying to piss off any of you tourney guys.
I think it would be cool, though, and put some emphasis on trying to complete everything at least once.