> If I remove it it will complain that it does not exist ...

only if you remove it from freesurfer's subjects directory

> I think it would be great to have one example that works like our examples.

just for create_default_subject? It's just one command :) The only reason the error happens is probably because you are using the mne sample dataset which already has fsaverage or another subjects_dir which already has fsaverage. You can always go with the one you already have, you might just not have t