I'd love to see users be able to (if enabled) redirect some of their email based on a set of rules to a re-direct address. Users can redirect now, but its an all or nothing proposition. That's the meta, here's a specific example of why we'd like it, but there are lots more reasons this might be cool:
We get a lot of spam email at several addresses that are also used by our clients to reach us. We do minimal spam filtering for everybody, but try to excise the spam for the high-volume legacy accounts in the email clients (typically Thunderbird). We've also been using these clients (with a plug-in) to redirect mail from known clients (at known client domains) to another account that is checked often by PDAs. We're going to start doing this select and redirect with a set of rules at the administrator's panel (with rules that limit the redirect to only the high volume accounts). It would be a lot more convenient (and make a lot more sense for us) to do it at the account level through the account-level web interface. The existing code (or even a more full-featured subset) would work great for client use.
Essentially, all the parts are already there, but not in the right places.