I wonder whether it will have a flat namespace that everyone competes over or whether the top-level keys will be user/project identifiers of some sort. I hope the latter.
But you can if you want have a non-flat namespace for imports using PEP 420 – Implicit Namespace Packages, so all your different packages "company-foo", "company-bar", etc. can be installed into the "company" namespace and all just work.
Nothing stops an index from validating that wheels use the same name or namespace as their package names. Sdists with arbitrary backends would not be possible, but you could enforce what backends were allowed for certain users.
import io
from contextlib import redirect_stdout as r
b=io.StringIO()
with r(b):import this
print(b.getvalue().splitlines()[-1])