It would be nice to bring some of the combat out from the keeps, resources & scrolls and have some open battles between keeps.

I suggest having an NPC convoy pushing carts with siege materials, and maybe a ram, from an owned keep to an enemy keep. Once there they will setup at the front door and attempt to bust it down. Obviously NPCs wouldn't be hard to counter siege, so players will have to escort them and help with the siege. Alternatively, you can scout for enemy siege convoys and attempt to ambush them and wipe them out before they get close to your keep.

I'm not sure what could trigger a convoy to form and leave for an enemy keep. Perhaps it could be tied in with the resources, so if you have all the upgrades it triggers the convoy. Killing a siege convoy NPCs could give you drops like the wandering Siege Merchants do currently.