It will be interesting to see what they do to make it really unique and worthy of a sequel. Trackmania United is already pretty awesome and no slouch graphically, so I wonder what improvements they are going to add. I read they will allow modders to import geometry into the game, such as custom track blocks. That will be kinda neat, but make online play a little...slower. Current TM tracks are a few KB in size so having every server full of custom tracks was no problem for anyone.
I still want to see what they are doing for Questmania and Shootmania.