Under the GPL license, the source code is GPL, but the original assets are not. Let's develop an FPS game in which we will reuse Doom, Descent, Build, and any other engine source code we need, and release the source as required, but keep the assets proprietary. I already have a C++ OpenGL engine I worked on for 5 years (MIT license), so I can help with some of the basics such as cross-platform compatibility (on C++ level), fast sprites, sound and geometry math. Does anyone want to help me port the Doom 3 engine to Android, then develop a sector map editor for tablets?