All modteams are welcome to these, I’m not going to sue you for IP theft or whatever. 
Serious stuff:
Control over how much health everyone has. The one gripe a lot of people I’ve spoken to have re ET is that you die far too quickly, so it should be possible for server admins to set things up so that the ‘base’ health level is, say, 200 instead of 100.
Everyone gradually healing (like medics do now, but slower). Once again, admins would have control.
Better behaved MG stands - You should be able to use the MG stand while standing or crouching. When you try to use it, you swing the MG down from 45deg upwards to 45deg downwards. If the stand hits something on the way, it rests there. If not, you just go back to using the MG as normal.
This would allow players to rest MGs on the tops of walls etc, and not have the floating stand effect you can get now.
The ability to grab a gun lying on the ground regardless of whether it fits your class (except maybe heavy weapons). You wouldn’t be able to use the special features of it, however (only coverts could use scopes, only engs could use rifle nades), and it’d have a wider spray (as your char whould be unused to it).
Fun stuff:
Grappling hooks. Worms ninja-rope style. Swing from building to building, firing a panzer, anyone?
More, different guns - I’m thinking shotgun (two rounds per clip, slow to reload, each shot is a fairly wide spray of about 20 bullets) and minigun (takes a second to warm up before firing and slow down again after firing, slows the player down like a heavy weapon, no reloading - it draws straight from the ammo stock).
The ability to carry multiple knives, and throw them.
Just some random thoughts. Hope someone likes them enough to implement them. 