It would be great to have better integration with the Gnome desktop. Email accounts, calendar and contacts added to Gnome should be automatically picked up by Thunderbird.
It would also be great if you could have it so that events added via Thunderbird are visible in the calendar drop down in GNOME, as well as having it so that when you click on an event or create an event via the GNOME drop down, it opens it in Thunderbird
Evaluating Evolution vs Thunderbird for use on GNOME right now. Thunderbird looks a lot more modern, but needs to be able to talk to GNOME system contacts, tasks and calendar.