Support for roster sequencing
Gajim ought to support XEP-0237 for rosters. It allows the client to cache the roster offline, and only fetch a new one when it has changed on the server.
This would make for a significant decrease in login time for people with large rosters.
Since I'm not aware of any release servers supporting it yet, I can supply a test server on request. Gajim would also I believe be the first client to implement this feature too :)