This feels very niche. Typing the \ at the end of multi-line macros is something many editors help with (and forgotten \s are visible in syntax hilighting.) Binary inclusion is frequently just a small build script. Deferred resource release is already available as a GCC/clang extension, so it's only for less capable compilers as you say yourself.

That leaves the @ operator… which seems useful for specific cases (e.g. graphics rendering), but I can honestly say I don't really feel a need for that in the codebases I worked with.

But. Let me offer some suggestions for extensions! :)

An improved preprocessor would be massively useful if it had features like:

- enumerating struct members in preprocessor macros (something like:)

  #define MACRO2(typename, fieldname, arg1, arg2) ... arg1 arg2 ...
  #define MACRO(typename, arg1, arg2) something { #each-member(MACRO2, typename, arg1, arg2) }

  struct bla {
    int a, b;
  }

  MACRO(struct bla, x, y)
  /* => something { MACRO2(struct bla, a, x, y) MACRO2(struct bla, b, x, y) } */

- same, for function parameters

- and last but not least, macros consuming { } blocks, e.g.

  #define MACRO(...)#{name} something something #{name} something #{name}
  
  MACRO(1,2,3) {
    this block is consumed by the macro and placed in 2nd and 3rd #{name} (first one just signals the macro consumes a {} block
  }

...oh and, almost forgot, "extensible" macros:

  #define MACRO(...) abc

  #ifdef MACRO
  #extend MACRO(...) def MACRO(...) ghi
  #else
  #define MACRO(...) def stub ghi
  #endif

(also, macros that can define other macros. Like _Pragma() => #pragma, _Define() => #define)