I was looking into making this myself, but could not find any means of controlling mobs such as creepers/zombies. Basically, my idea was that of allowing users to run some command or interact with something which would allow them to control a creeper, zombie, or skeleton. A creeper would be able to break down walls for the rest of the opposing force to go through. Zombies would be slower, but are able to withstand alot of damage. Skeletons would be able to pick off players that are defending from a distance. Once the mob that the player is controlling dies, they are returned control of their character and can send another mob to attack. The defending players set a "spawn" point in their base to come back as when they die (make it a single spawn point to prevent exploiting). As you can see, once you allow the controlling of monsters and their events such as explosions and attacking, this mod is fairly simple to make. I'm also open to ideas on how to implement this myself.
