It would be nice to be able to manage certificates and usernames from the mumble client. You would probably need new permissions (admin-self and admin-others) although it could possibly use the same permissions as registering (would need to look into that more carefully). Some things that I think should be possible are adding a revocation of a users certificates, and possibly changing usernames.
Adding and revoking certificates is necessary because of expiring certificates and if a user wants to use different certificates on different machines (this way if one gets compromised you don't loose your account, you can just revoke that certificate).
Currently if you have passwords you can add new certificates but there is no way of revoking them which, in my opinion is a security issue.