Mappers can already do this. They can use various of statics to create such supply depots without any sort of problems. However, to make them feel important and act out as what they are meant to be is all upto mappers vision of the maps design. If they plan that an ammo depot is supposed to rearm everything from handweapons to tanks and planes, then they simply include that. It's just, not many mappers have done this in the scale that it would actually be part of the gameplay of said maps. Many maps have these depots already, they just serve no other purpose than rearming your weapons, they do not stand out that well since they arent huge installations but vary in sizes.
For instance in a Bulge map you could have german objectives be supply depots.