I'd like to see something akin to VC's include directory dialog that would let us add paths, say for instance the Platform SDK dir, WTL dir, etc. These would be searched for headers that would then be used to create ctags info.
Admittedly I don't know how the ctags stuff really works, but I think a customizable solution like this would be better than an .api-style solution (at least for C/C++, anyway).