Some feature ideas once everything is nice and stable:

User profiles for each identity. Just a text field users can fill with information like the Bitcoin address, Dwolla ID, mailing address, etc. if they so choose.

PGP integration for the messaging features (are messages public?)

web-of-trust style user rankings, but that's already been requested.

Minimize to systray + systray notifications for new messages, accepted trades, etc.

Some kind of remote API (i was thinking about android apps, but i2p's memory requirements are to great, so a front end is all that seems doable)