If I had the time to learn how to use OpenGL or D3D (I am partial to OpenGL, myself) in C++, I'd make an engine. The point is that I don't have time and anything I'd make would be somewhat primitive Quake-3 level. I did have a nifty engine going a while back though... it just wasn't practical and I didn't have time. I had almost finished JPEG texturing, too.
What this will probably come down to is me making an Unreal Tournament 2k3 or Tribes 3 mod. They share the same engine, but Tribes 3 would be more suited to my exact needs, with a lot of the work I'd have to do already being done. When is Vengance supposed to debut, anyway?