Helm's Deep is already built and we have no part in the construction. Not so for many other places that could find themselves under siege! Nothing's happened in Forochel for awhile now, right?
I would love to see areas that we can build up and have our upgrades have real impact on an epic battle. Repairing the wall, building the towers, getting the forge cranking out weapons, whatever we've built is going to be helping us stay alive. There could be slightly different versions of the instance based on what major structures are up on the landscape when we launch it, or the landscape upgrades could mean we have extra soldiers/barricades/weapons to work with when the battle begins.
Obviously to keep the fight from getting easier and easier the enemies would be adding new things to their attacks based on how many upgrades we have. Sallying out to kill the stronger and stronger enemy commanders or capturing their supplies could provide the currency to spend on defense upgrades. The end goal would of course be to have such a strongly defended position that the enemy tries to break the siege with an oliphaunt attack.