So far, the 3D titles can be analogized with the sidescrollers pretty closely. The first one has primitive graphics but good jump physics, the second one introduces a weird new mechanic and is the black sheep of the series, and the third one has lots of power ups and goes on a whirlwind tour of a big place -- the entire Mushroom Kingdom; the entire universe. If the analogy continues, we should expect the 3D Super Mario World, basically Galaxy 2, around 2011 (half the length of time between the previous games, like Super Mario World). It will probably be similar to Galaxy in graphics and controls, take away a few of the suits and add one or two more, and focus on a nominally smaller but much more fleshed-out location. There's a pretty good chance it will be set in the Mushroom Kingdom, since none of the other 3D games have actually been there yet, surprisingly, other than the Grand Finale Galaxy.