Except for small/simple projects, I almost always adopt Fortran submodules now to structure new projects. This is primarily for the parallel and incremental builds that they allow.

I realise this may be low down on the list of project priorities currently, but I think it should only require an update to the dependency tracking whereby submodules depend on their parent modules as if they use them.

Much further down the line, I assume fpm will aim for some kind of parallel build support?