If we plan the architecture well, or are clever with compiler switches I think we could emulate a system similar to the gallium developers. In other words have multiple backends which target different systems which can be worked on by people with expertise in that field. I have a system like that for the Prometheus library that when compiled for linux uses my X11_Window unit, for win32 uses my WINAPI_Window unit and for anything else uses my SDL_Window unit for event/window handling. Each unit basically translates a Prometheus call into the relevent OS level calls. That way, as a user I can do

Code:
CreateWindow(640, 480, 32);
And the compiler will automatically compile the appropriate code for the target system. If I recall this is similar to the gallium project as they have an implementation for each standard and that plugs into their actual driver code. So they implement OpenGL once, OpenCL once and even D3D once and to add enw hardware they just write the hardware specific stuff in the layer under.

Also, I dont have delphi so any code I would work on would be FPC. I also agree with the decision for using 2.6.4 as the current stable version. As for graphics, OpenGL3 and 4 seem like a good choice. As most of my work has been with OpenGL 2.1 I'd be happy to convert some of my implementation from the Prometheus library to supply a 'compatibility mode' for the engine

Edit: Forgot to mention this, but the prometheus code has texture handling, FBOs, context creation, extension loading, basic 3d support and I'm working on an HLSL implementation . It uses Benjamin Rousseaux's rather excellent TGA and PNG loaders for image loading