Some relevant things:
Nim already expects global Nimble packages in /opt/nimble/pkgs (https://github.com/nim-lang/Nim/blob/devel/config/nim.cfg#L49)
This issue has a lot of relevant discussion https://github.com/nim-lang/nimble/issues/80
Ah didn't know that. Still I propose /usr/include/nim for packages that are handled by your system's package manager and not nimble.
I figure nim doesn't need to look there by default, just encourage packagers to add it to the nim config. My meson work doesn't really use nimble at all, and I indent to disable nim's nimblepath features. (along with {.dynlib.} for import.)