is there currently no way at all to save bank data to an external server database? if not, i think blizzard should consider implementing a means of doing so.
now of course this would bring up the issues of overloading the server and everything but there are EASY work arounds to that. (like requiring a server connect and password to be entered into the chat, you could also pass username and password for authentication/character selection etc)
this could allow multiple servers (hosted by small communities at their own expense) but keeping the map / game the same. (maybe allow global variables set by the external database to allow certain things, like dbl xp bonus stuff like that)
another incredible feature would be join in progress.
both of these features would be just awesome for your map, as well as many other map ideas out there.