I would enjoy whatever they do with the fighting engine as long as it's good. If they made UMK3 in 3D I would be happy because I still love that fighting engine. But if they made something 100% new and different and it was deep and balanced I'd love that too.
I think part of the problem is there is a different concept of what a finished product is between MK and games like SC, TK, VF, or even other 2D fighters like most of the capcom games.
MK games are released at a level of finish that would probably be considered early testing for other fighting game series. And, despite that, they sell a lot of copies of the MK games. So I don't see a huge change coming in the future. I think we'll still see unfinished gameplay engines released. But that's just my cynical point of view because I've been let down by MK for a long time now, lol.
My hopes for MK8 are building somewhat though. At some point in conversation with someone (probably Shock) I mentioned a long time ago that I hoped they would just use an existing engine for the graphics so that they could focus completely on the gameplay instead of spending time making the graphics engine too and that's what they are doing with MK8. Hopefully that means they actually do spend more time working on the gameplay.
And even if MK8 isn't as finished as I would like, I hope MK9 is an improvement.
MKDA through MKA they didn't focus on fixing problems. They just kept adding more broken crap for the most part.
MK1 through UMK3 OTOH, each increment fixed elements of the gameplay, which is why UMK3 turned out good. It was a steady progression in quality through the games. Plus, because they were arcade games, each game got revisions as well.
So for the 3D games to be successful I think they'd need to either spend A LOT more time testing and polishing the gameplay, or they'd need to re-adopt releasing revisions and incrementing the same engine across multiple games.