Upgradable guards and siege equipment, perhaps at the cost of branches/snow/soil
Add in the siege/player boats
Give tanks more to do at sieges, perhaps some sort of ladder system at the cost of some sort of resource, that is perhaps built up during the course of the siege. The end result being an open climb point that attackers can use, the defenders have the option of destroying this equipment, if they can get at it.
Increase the HP of outer gates to accommodate a more fluid pounding where taking the walls rather than bashing the door in becomes an option.
Add the option of shoring up a door, sort of like rez sick repairing, half the speed, twice the resources.
See if the condition for a keep being in siege can be looked at, could it be changed to X% of damage has to be done to the door (even as low as 1%) or 2 guards have to die in any given period? ... say 2 guards a minute.
Maybe add in some sort of extra magic resist for heavy tanks so that they can actually tank damage for said wall fights.
Add in archer siege wrecking ability. Perhaps a self buff, 2 min timer, 10 min reuse, tied into bow spec.
Add into shield spec some sort of AOE guard ability for archer defence
Boost archer damage, or at least improve the damage multiplier a little.
Boost heavy tank 1h hand damage to make pure shield spec tanks an option.