Tremulous is a great game, however I would add these features:
Weapons:
~ sniper rifle
~ laser trip mines
~ remotely detonating explosive charge / grenades
~ small missile launcher
~ toxic gas canister
Buildings:
~ research facility- e.g. allows to upgrade buildings (e.g. reactor- more power; less energy consumption; larger energy range for buildings), weapons (bigger ammo clips, better accuracy, more damage, higher rate of fire), stronger armor
~ mounted gun operated by human (e.g. gattling gun)
~ flamethrower turret
Additional features:
~ RPG elements- upgrading human stats in armory (e.g. accuracy, speed, carrying capacity, endurance / HP)