It would be nice to see an AJAX interface for external users for notebooks and briefcases. Although the one they get is rather clean cut it would be outstanding if they were able to choose between the normal jetty interface as "basic" and an AJAX interface as "advanced."
Will we see an interface that allows external users to upload to briefcases any time soon? That would be a great addition. Not all users will have admin rights in order to grant access to the Zimbra server, it would be great to allow external users to upload. The admin can grant a user's ability to allow external user uploads, and the user can set a quota for the external user. That sort of thing.
Also... It would be great to be able to manage external users in your address book. Possibly giving them passwords to shared things.
Allowing other protocols or a wiki howto on jabberd use in order to allow users to access their external Instant Messaging programs (yahoo, aim, etc.) would be an interesting addition. I know Yahoo owns Zimbra, but they can still show some compassion if it is to truly "collaborate." If a one stop place to conduct all internet business is what Zimbra's aim is, then these few features would be a great addition. If a person can share files and documents, chat on all his/her Instant Messaging programs (protocols/services) and not worry about requiring someone to have an account on the same server as them... I think that would be a goal met. It would be something that most people would not be able to do without.
Overall Zimbra has been absolutely amazing. Just trying to poke and hope that my comments will allow it to be absolutely perfect.