Would it be an idea to incorporate something like siege battle in some future expansion? With stuff like walls, siege equipment for both the attacker and defender, maybe something along the lines of traps, weapon emplacements (like a turret in a small bunker for later ages).
I don't know how much this aligns with the idea of the game, but I enjoy the odd siege battle every now and then in games (like Total War).
Good news, there are actually rules for defensive emplacements in the 2nd Age! They require you to field one or more models with the Engineer X Ability, and you can put down a number of emplacements equal to the combined X of all Engineers on the field.
These emplacements have both a point cost and an Initiative cost. That is, they raise the value of whatever deployment zone they're placed in by 1 or 2. You have the choice of either Sandbags or Trenches, the latter costing more but offering far more protection for a defensive fight. Each segment is up to 8" long, and Sandbags can be placed as a half-circle for reduced cost.
In terms of weaponry, Humanoids are the kings of siege warfare. They have the greatest Armory access out of all the Domains; this includes massive bore cannons and Multiple Launch Rocket Systems that can fire incredible distances with devastating effect. Don't sleep on the other factions, though, Biests have a nasty habit of looting battlefield equipment, and the Reptilia have a few tricks of their own.