I tink that you shoud build the levels from Models as from Blocks. I know, the Manual says otherwise but with models you can do thinks more detailed.
And i thing you cann do more "cool" things. A Spiral for example. The game just need a little speedup and some things like hovering platforms and magnetic railes, like in Marble madness for Wii.
I would make a selection of platforms, ramps and pipes and so on, so you can easy combine them to many levels.
The game has potencial for sure, you just need mor time. Keep going and in a few week the game looks way better. If you ever would see, what the very first version of MM3D looked like, you would surely throwup ;P