The thought of developing a 3D engine in itself is a herculean task and staying motivate to develop a 3D game in the midst of it seems almost impossible. I really don't want to spend the better part of a year or so developing something that may not get finished or when it's done all I have to show is some renders of a level or shading algorithm.
Nope, what I want is to produce some small games. 2D right now is the ticket, and finishing something has higher importance. The juice for me has always been developing the logic around an engine and of course creating the game graphics.
Once I get a decent 2D engine up and running then I'll evaluate the prospect of creating a 3D engine on top it but then again that depends on how much fun I'm having doing 2D stuff.
3D is cool, and I've seen quite a few engines with very nice features but I've also very few 3D games.