Something which I just thought of reading the feature list was some sort of confirmation mode. When someone joins your server (*before* the level downloading process if possible), have it freeze his joining process for x seconds or something (just not sending packets back or something? if possible), and pop up a message to the host asking him to /confirm (or something of the order) the incoming client. Good for invite-only servers of all sorts.

Btw, Koth concept THIEF :-o