Here are some suggestions:
1. Team members shouldn't be disbanded on teleport (implies that team members are on the same district)
2. Friends/Guild Members list should contain a button to teleport to friend and to invite a friend.
3. Guilds should have a default district.
4. Automate "looking for group" to help single characters to find a group
5. Assign a unique number to mission so that missions can be easily identified accross languages.
6. "Quest updated" message should be displayed upper part of the screen. So that you can't oversee it!
7. Blips to indicate the direction of a fellow team mate or ally that is very far away (outside of the visible compass)
Some suggestions are from other people. some are mine.
Here are the sources:
[1] mine
[2] mine
[3] not mine - don't have link any more

[4] http://www.guildwarsguru.com/forum/s...ad.php?t=12815
[5] http://www.guildwarsguru.com/forum/s...ad.php?t=11364
[6] mine
[7] http://www.guildwarsguru.com/forum/s...ead.php?t=9843
Maybe we can collect more GUI suggestions in this thread.