When I look at map at Cyrodiil and see so many keeps, see resources and siege merchats there...
What if siege merchnat has limited stock to sell and the only way to replenish it would be Convoy which will be going from keep to keep (rules like transitus shrine - you break the link and the convoy won´t reach keep which is cut off, convoy would stop at place until the link is restored or the convoy is destroyed).
When the convoy arrives it will deliver new stock to merchant. It would make repairs of keep harder, becasue right after capture you won´t have resources to buy from merchant. Resources around keep would replenish some stock also but very low numbers.
Now the convoy would be made of NPCs (strong ones and their number would be like 10-12).
On quest boards you would have attack and defend quest for conyos. You could escort yours or capture those of enemies . Escorting would mean that you enter their circle area and you would have to stay in that circle at any cost. If you leave you would get mission failed to force you really escorting (area have to be large enough so you are not range attacked in a way that you have no room to rush and kill the attacker.
Attackers will know the road where the convoy is, but they will have to locate it and have the opportunity to plan the attack. You will know to which keep it is going.
Reward for these quest would be larger portion of gold, so those who play PVP only and strugle with gold would have alternative to do PVP and earn gold as well.
this is just the starter idea and I am sure people are able to come up with stuff that would tune it up

It could even ignore the limited stock and be there only as quest purpose with no further impact on war.
I hope it makes sence as I wrote it in rush during lunch break