[sfx1999]

My advice:

Tweak the Quake 2 BSP format to get a similar functionality as Half-life 2 maps. Add displacements, 24-bit lightmaps, 24-bit texture support (or variable bit), more clipnodes, etc.

Use the OpenDE physics library. There are less restrictions on it than Tokamak, and it can be used either under the GPL or the BSD (credit clause version) liscense. This avoids some problems with the GPL.

Freetype for fonts. You can use the GPL or the Freetype liscense.

OpenAL for audio. LGPL liscense.

Rendering is going to be tricky, because some OpenGL cards don't support shaders that are supported in Direct3D. I would suggest using Cg and OpenGL, because of the problem with GL_ARB_fragment_program. IIRC, Cg will automatically configure register combiners.

For netcode, maybe you could use Quake or Quake 2. GPL liscense.

Get a versatile image library. PNG and JPEG support is a must. You should use JPEGs for textures, and PNGs for things like normal maps, because they need to be lossless.

To handle input, it would probably be best to use something like SDL or GLUT. SDL is LGPL, and if you use Freeglut, it is under the X-Consortium liscense.

To make maps, it would be best to use something like QuArK, because Hammer has restrictions on it. It says you can only use it for Half-life maps.