Faction Castles - The factions should have Flag models they can Hang around in their castle. (They shouldn't be able to just spawn their own castles)
About Castle sieges: Builders Guild, They should add that faction, Or it should be a Subclass for traders so they can sell Siege weapons to Faction leaders. (Let the faction leaders DUP the stuff)
Recruits - If there's a massive faction war going on then the Factions should be able to Abduct random peasant to fight for them. Giving them recruit armour(Should have a special Armour Depening on faction colour), And it would be fun for newbies.
Use of NPCs - People should be able to buy NPC animals.. (In a cage with Wheels Maybe?) And then release them inside the enemie camp. ( Would be awesome if someone Set a T-rex loose inside of a castle!!)
Trader class should be very awesome if they could have Wrangler and Builders Guild Subclasses.
Check Summoning Thread for animals and i think there was already a denied engineer's guild request. and aint we a little too few for Seigeweapons and castles
I mean the only once that would have em is Duke and Baron
Hmm, I dont see how all this will fit into it.
Good idea though, maybe you could adapt the idea a bit more?