I think it would be a reasonable assumption that most people have `fsaverage` in whatever directory is returned by `_get_subjects_dir()` in `mne.utils`, and if they don't, it would be reasonable to make them supply the correct value from the command line. This would be most consistent with how other parts of mne-python are coded (that is, taking `subjects_dir=None` as default and using `subjects_dir = _get_subjects_dir(subjects_dir)` to get it).