If a some_module defines __all__, dir(some_module) should only return what is in __all__. This is already a mechanism that Python provides to specify module-level APIs. Currently, dir(some_module) returns some_module.__dict__.keys().
The concern with this enhancement is backwards compatibility. It is conceivable that some library's code would be broken with the different return value of dir(some_module). However, it seems unlikely that any code, other than tests, depends on the current behavior of dir(some_module).
If __all__ is not defined in some_module, the old behavior is preserved.