I think a 2d style platformer viewed in 3d from behind like the original Crash Bandicoot would be cool to see in DB, often planned such a thing myself, just not got round to it yet.
A good Mario64 clone would be flippin brilliant, but the move set would be difficult to implement with collision in DB, because most collision engines don't like fully 360x360 rotation.
As for level ideas, well you can't go much wrong with the traditional platformer themes, minecarts, snow, beaches, lava etc. That said something original would be good to see.
Personally I wish more people made 320x240 "D sidescrolling games with well drawn graphics and well though out levels. Perhaps a Sonic Clone, with decent floor2wall physics
The forum user formerly known as Kangaroo2.
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