1. Decay of the abilities
Decrease the level of the abilities if the player plays (for instance) continuously for a week. They are decreased of 1 level and half of the points are given back to the player to be spent.
2. Custom key-binds
I wish to have an interface like the classic one to bind the keys easily and maybe also to set weapons priorities only for ballistic gametype.
3. New wave
One of the few times I played UT3, there were a custom gametype named "Jurassic Rage", also present for UT04; I would like to suggest you the monsters used.
Crazy Ideas
1. Remove some weapons from the Invasion gametype
I would remove laser and plasma weapons and leave only the more classic ones that reminds real weapons (AK-47, m46, m50, redwood and so on).
2. Health when revived
Get the health back to 100 when revived.
3. Defense Invasion
It was a custom gametype where the monsters spawned only from one side of the map (there were 3 big maps) and then they attacked the base of the team. Maybe you will like the idea

I would be glad to hear your thoughts about this, i LOVE invasion.