Super Mario 128 will be the true sequal to Super Mario 64. If you want a better idea of how good it should be, think of the Mario Brothers games for NES.
Super Mario Brothers (The first real Mario game, not counting those arcade games) *Relates to Super Mario 64
Super Mario Brothers 2 (This one was different and the least favorite for many, but still good) *Relates to Super Mario Sunshine
Super Mario Brothers 3 (The greatest of the three!) *Relates to Super Mario 128!
This will not be Pikmin meets Mario, this will not be 100 Marios, this will be the next real adventure. It won't be rushed, it won't focus on objects and it won't be buggy.
(I am not saying SMS was bad, just saying that it will be the worst out of the 3 most recent games. Or at least I hope.)