I'd like them to merge the best of the other 3d games.
The exploration of 64 and sunshine, the off the wall creative level design of galaxy 1 and 2, and the precision and polish of 3d world.
A subtle but moving plot would also be nice.
As for new mechanics. Games like galaxy 2 seemed to have new mechanics every level that would suitable to base a whole game around and I want that kind of creativity.... but what mario really needs is a portal gun.