I was very happy to see DOC on your plan since it reminds me of my old favorit: HEXEN II Siege.
Well, that game is now more than 5-6 years old, but the idea was great.
see: http://www.hexenworld.com/siege/
You could have 1 or 2 (DOM style) Castles with destroyable walls, gates and many different ways to enter (and to defend!).
In the original mod you could use catapults to hurl yourself over the castle walls, or swimm through the sewer (full with nasty animals) into the courtyard, but you could also dig a real tunnel. The goal was to get to the enemy's throne chamber and get the crown.
The defenders could use stationary bolt throwers to get some extra power.
CUT2's Melee system and the crossbow is perfect for this atmosphere!!!
Please Please Pleeeeaaaase!
Take a look at the website and try to incorporate these ideas in your project!
Speaking of Invasion... Has anyone thought about a Helms Deep (LotR TT) style map? With movable siege towers?

Many thanks in advance!
Endre
Ps.: Sorry for my english...