I would like to suggest a new item: the backpack or ammo satchel. The backpack would act like a battery pack, only it allows the player to carry 2 extra magazines or more ammo for weapons that are not energy based (eg Lasgun, Pulse Rifle, etc). Naturally, this item would be placed on the back and could not be used with the battlesuit, battery pack or jetpack.
I think it would be nice if battlesuits could carry more ammo, too, but maybe not as much as a backpack to balance things out. Since the Chaingun is the weapon for the battlesuit, it kind of bothers me that you can carry so little ammo for it while in a battlesuit. You often run out and have to run back to base. This can cause problems during alien base raids.
Maybe Battlesuits carry more ammo for weapons like the Chaingun and Flamethrower, and the backpack carries more ammo for weapons that use magazines, such as the rifle and shotgun? Just a suggestion. :)