Thats what my suggestion is.
Update w32api (already being done) such that both ReactOS and WINE can be
compiled with it (and the platform SDK headers too) and move anything that
doesnt go into w32api from the ReactOS and WINE existing "API" headers out
and into wherever they fit.
From there, ReactOS (at least) can drop all the mess of win32api headers
there is now (i.e. the stuff in \include like structs.h and friends and the
WINE stuff) and switch to w32api full time for the entire project.