For an MMA game to be successful it needs:
To have fast and on point controls.
Needs to re-think, entirely, how grappling and clinches work.
Stand up is pretty much perfected at this point. (See: Tekken or Virtua Fighter) It just needs to be more realistic.
Career mode worth fighting for much like MHBestEver said above.
I think the achilles heel is the grappling/Clinch system. It should be intuitive and they really need a BJJ expert to go in and show them how important balance and strength in these areas should work. This engine needs to be revamped. Currently, in most games where you grapple/clinch it becomes a mashing war instead of a balance/strength war.
I suggest they develop first a way for the fighters to initiate a clinch or a take down and as soon as they are on the ground or in the clinch a different mode would start. For instance...
On ground after shoot and in full guard:
Now the fighter (agressor) has one of three sets of options. Punches (GNP), say Square and Triangle on a PS3 control. Position switch, either left or right you can fight for half or even fullgaurd. This will leave you open to either getting reversed, or to strikes fron the bottom. Submission, you can try a submission attempt that is possible for this position on the ground by holding say, L2 and hitting a button combo. The rate of success of getting the submission depends on your strength, your endurance, some button mashing, and your position.
THe bottom person would be used much the same way, except, with more options to defend, and strike rather than submit.
So many factors in MMA to consider. But if they can make such a richly technical game like Vitua Fighter, that is not only fun to play but very much depends on skill rather than luck, then I think THQ can do the same.
Personally, I don't think THQ is up to it. I see a highly visual realistic game, with little to no gameplay in sight. Unless they can tackle the issues on the ground and in the clinch (which needs to be understood before your a true MMA fanatic).
If THQ needs people to help I got plent of ideas.
Top 5 Fighters:
Antonio Rodrigo Nogueira
Mirko "Cro Cop" Filipovic
FIGHT PREDICTIONS (Since June 11th, 2007): 10 - 2