Now, the mod, as it currently stands, has very few large underground structures. The nearest would be the Dwarven Mines. Orc "dungeons" are the same as vanilla spawners, and incredibly difficult to find, with little reward. I am simply proposing more dungeon-like structures. Doesn't need to be an actual dungeon, but perhaps generated in the same way as other dungeon mods-piece by piece. The best thing about them, is that there is absolutely no exterior work to be done, and the rooms can generate mostly independently, only limited by how they fit with other rooms. They also are not carbon copies of other structures because they each generate randomly.
Most factions should be able to have at least some form of larger underground structure. Anything from tombs, to eating halls, to barracks, to goblin-cities.