If you would like to take on something that is genuinely lacking from C, try creating something equivalent to C++ templates for C. I'm not sure that it can be usefully accomplished using only the C preprocessor. The C preprocessor is extremely underpowered. So it might require its own preprocessor.

I wouldn't make them exactly like C++ templates. I'd just focus on the useful things that you can't currently do in C, like write a qsort()-equivalent that doesn't suffer the performance penalty of callbacks. You can almost get there with macros, but not quite -- it would be ridiculously ugly at best.