Perhaps instead of drops, the bunker has the normal 4 holes of defense, but there is an underground storehouse that allows you to store stuff via the antiquated storage crate inside. After certain kills, certain doors unlock (or become destructible via Lua) that unlock weapons, tools (concrete sprayers, etc.) and actors. Here's a picture of what I mean:
Figure 1.1: What I think. Please excuse the poor drawing, I did it in a few minutes.
As you can see in figure 1.1, the bunker above is still the same, but underneath, there are 2 upside-down tall doors. that leads down into the store room, where you can access the unlocked weapons (they spawn 60 seconds after you take them, and only 1 can exist in the chamber at any time). It needs a few rows to accommodate all the weapons and actors that can be unlocked.The doors are pretty much invincible until you unlock them by getting enough kills, which will make said door gib, leaving the contents unharmed. The actors are unlocked at intervals of 100, starting at 100. The storage crate is at the bottom, and the warehouse can be a last-ditch defense point. This will only work properly if you choose not to give the players funds.
The bottom-most coalition soldier is buff.
To bring the weapons up/down, you get a robot 1/coalition light that you can either use to fight, or as a courier of weapons and tools. The upper bunker is in scale to the current bunker, so the store house can be made out of Vanilla bunker modules, except the doors.