I had an idea a while ago that would go something like this, it would take a lot of moderation and some specific, probably system intensive, maps though. Essentially, it goes like this.
Attackers Spawn - Empty terrain, woods, roads, etc. - Village/town - Some more terrain for distance - Earthworks, trenches, etc. - Fort.
The defenders move out of the fort and take up positions in the town, they're allowed to use sappers to build barricades. After this, the attackers are let loose. When a regiment is wiped out it must fall back to the next defence, in this case, the trenches, and when the attackers capture the town, they move on to take the next defences until they get to the fort, which they must capture to win, obviously.
Very much based around waves and different places to defend.