Here's a list of great suggestions of mine:
Repair gun as a purchasable sidearm. Make them really expensive so that it doesn't quite defeat the purpose of an engineer (plus, engineers have remote c4).
A chat box that doesn't disappear. It should be visible inside PT's (like the original Renegade) and in ESC, too.
Voice communication. Duh.
Easier private messages.
Easier donation method.
Settings in the ESC menu. This especially helps with mouse sensitivity and volume and things like that. That would be the biggest improvement, I think.
Teach the harvester not to run over other vehicles that are in the way. This is especially the case in the ever-defending NOD base in CnC Fields. The harvester just runs right over anything that's in the way. Or pushes it. Or worse. It's really annoying.
Big problem with Fields is that GDI can send air strikes to the runway at NOD's base.
Also, air strikes are way too powerful...
We need tech building indicators at the bottom of the screen.
More damage indicators for direction of shooting and severity of shots.
More tie breakers... Things that are competitive and difficult to get, but give an advantage to a whole team. Not such a great advantage that it can be effectively used in normal situations. Just enough to help when both teams are failing.
How about some naval maps? New vehicles? Boats for NOD. Hovercrafts for GDI.
While you're at it, add on to the air vehicles. Jets? Only for particular maps of course.
I don't know. I'm just throwing ideas around. Some of these are obviously more important than others...