Theres a few suggestions:
* Melee weapons sould make sound wen attacking walls
* Maps with destructible architecture. Aliens breakings walls to enter the Human base can be fun. Aliens already can move trough holes, ... this map idea can be really fun, imho.
* Better smoke for jet, the actual one is very unrealistic.
* Better explosion fx for grenade
* Gibs for gibbed aliens/humans on explosions
* Ammo draw on weapons models?
* Better turret model. Actual is sligty lame
* Forest/whatever alike map
* Grenade launch animation? (hand?) maybe not a good idea.
* Blood stains for aliens and humans. Huge blood stains for humans dying.
* Smoke puff on gunshots impacts?
* Draw a "Mars" map, and place official maps on this map. And write a history about a alien infestation. Because this game has that good atomosphera, really will be fun to have a plot or whatever context other than the senseless, pre-puber, counter-strike-ish, simply fight your enemy.
* Seeks for more mappers. As posible create another "oficial" mappack, so people dont get tired of actual maprotation
* Ability to move base witouh the need to dissaemble. Move mean you push, or "Get" stuff, and wall with it. Maybe with need of 2 people, maybe with a expecial tool or whatever. Actually the human "base moving" task is dificult and risky (Its that bad??)
I think the gameplay dont need changes, so this suggestion are minor stuff, mostly.
Anyway more gamemodes may not hurt:
* Civil War: Humans vs Humans
* Race Supremacy: Aliens vs Aliens
* VIP escort.
Future real hard enhancements:
* WIDEN maps, with dropships and ET:QuakeWars feel
* Support for Q2 maps, to reuse Gloom maps
* User made clan decors for buildings. (?)
* Maps with water? (?)
* Whatever