I'd like to see an eventual UMK9 much more so than an MK10. The problem with a full sequel is that every new MK is usually a clean slate. Most games feel like a completely new series altogether; look at MK3 to MK4, MK4 to MK5, even MK6 to MK7. Deadly Alliance and Deception are probably the only separate games in the series that feel completely relative to one another. Sure, everyone wants to see the series move forward, and no one wants each release to be the same game all over again. That said, I feel like NRS got the premise for MK9 right. Bugs and potential balance issues aside, the game feels great, but it doesn't feel perfect either.
As Skisonic brought up when Aris was bashing MK9, this is their first real attempt to make MK a competitive fighter, and while it has its problems, you can't expect them to get everything right on the first try. That's exactly why I'd like to see them release a UMK9 so they can perfect everything that went wrong: I'd sooner see them release much more foolproof version of MK9 than risk putting out a new game with more of the same flaws, due to too many core gameplay changes. To anyone who might disagree with me on this, just think back to when you were a kid: Do you remember how badass UMK3 was? Wouldn't you say it was a huge improvement over MK3, with all the fine tunings that MK3 didn't have? The thing is, MK9 feels like MK3 to me: It broke new ground, but it's not without its flaws. I'd sooner see them take the time to correct these for an expanded edition of MK9, rather than start all over again. They could potentially work Shinnok into the story for everyone who's been asking for that, they could add some new characters/new moves, they could rebalance anyone who might be an issue, the list goes on. Anyone here who plays Street Fighter could tell you that SSF4 was leagues better than vanilla SF4, both for the added content and the improved character balance. I'd love to see MK9 get the same treatment.
As far as I'm concerned, MK9 is the tightest original release since MK3, and MK3 got a UMK3 because of that. There's no reason MK9 couldn't get one of its own.