Before taking the task to modernize the whole library I guess the most important part must be fix some of the functions that don't work as expected.

As demonstrated with "Doom3", texture mapping and vbo's fail to work properly. You can live without texture mapping with shaders but surely vbo needs to be fixed or further progress doesn't make sense for 95% of projects out there.

Also I suspect Stencil support is not ok and that is essential for modern shadow render code ( Doom3 fails at that ) and for example some of the tricks Scummvm does with OpenGL games (see Grimm Fandango drawing objects in bad order).

Also documentation about the intricacies of Tinygl could be helpfull too at the time of debugging.