I have recently been in the position of converting some gas source to Nasm, and 2 things that would make things easier are:
1) accept #line directives (treat them as %line?)
2) accept a command-line option that would specify a command to invoke a C preprocessor (e.g., "cl -E") ahead of any Nasm "native" preprocessing - or even instead of, if the -a is also specified
#1 would make life a bit easier when using a C preprocessor, while #2 would [practically speaking] give Nasm C preprocessing capability. Some of the obvious benefits of the latter are "support" of C/C++ include files and C comments.
