If doxygen, openal and freeglut aren't used, I would say "rip them out" instead of nulling them.

One might still want to build the derivation with OpenAL if they don't like SDL handling sound for whatever reason. I'd leave FreeGLUT as well. Since Doxygen presumably isn't used (but still recognized by the build system) I'll drop it from inputs.

In that case the nicer option is to add a useSDL flag (in the SDL case) and avoid passing the nulls.

It's semantically redundant, verbose (asserts, optional, ...), and requires one flag for every optional dependency, which doesn't scale very well.

I beg to differ. Let's say you use "useSDL" as an example, so you end up with

   buildInputs = [ some package and another ] 
  ++ lib.optionals useSDL [ one or more sdl packages ]
  ++ lib.optional (!useSDL) [ openal ];

That is far cleaner than having to remember which attributes to null for the various combinations.

Your example presumes that SDL and OpenAL can't be built at the same time. That isn't true, most emulators that have multiple audio/video backends allow to choose which one to use in preferences, including Yabause.

Ah, I didn't know you would build it with multiple options - then your way is definitely better than mine.