"Nintendo did no wrongs with Super Mario Galaxy, and I think in order to top it they'll have to introduce a new gameplay mechanic that changes the way gamers play Mario. In Galaxy that was Gravity, and the addition of new and radically different worlds that challenged the gamers to think in a new way. Sure, time travel is possible, but how would that fit into the world? That being said Mario is a franchise that can do practically anything, except adult content, and I suppose Miyamoto could always find a way to add it in. "
Seeing as they already did the DS game about time travel mentioned in this thread, it would "fit in the world" just fine.