This integration made for kyapanel project look it:
I belive to do it is necessary totaly rewrite a mail module to integrate with zimbra. A separated alternative module to make it. keep the original and can choose at instalation process. Can take ideas for new module mail from kyapanel project.
Acually ispconfig 3 have a complete solution with mail user interface integrated with round cube mail. You can change password, create a spam filter rules, vacation and spam level on user level experience using roundcube interface.
Make the simple things like zimbra. not all thing but basic things.