New (DE?) Troops: Siege Tower
Let me preface this by saying, that I think SC could add a new resource to the game. Preferably something that fed into a new troop family. I'm thinking Steel or Ore or something like that. The new troop family would be mechanical in nature, rather than magical woodland creatures. IF they were open to something like that, then the siege tower unit would be better suited there. Otherwise, there's only really space for it in DE.
SIEGE TOWER:
Very slow moving unit with major hitpoints but little offensive capability. The siege tower rolls to walls and then parks. Any unit can then use the siege tower to climb over the wall and then attack whatever is inside. They'd have a few drawbacks to keep them from being too powerful: Speed and one time use. Basically once they've attached to a wall, they stay parked until they are destroyed.
COST: 200 DE to start
HOUSING: 10 spaces
SPEED: 6
HP: 6000
DPS: 0 (perhaps destroy wall they were attached to when killed?)