Walls
Wall Choice!
I think Wooden Palisade and Stone Walls should be two seperate structures - wooden walls for cheap, fast defence and Stone for the more expensive, time-consuming alternative. Giving us a choice would be great. You could have Settlers/Villagers (with their limited space on the building tab) just build the strongest available (i.e. Stone if researched), but the option to build either with Settlers if there is space on their tab or certain military units (maybe Pikemen to give them more use, or even an Engineer/Pioneer
Wall Defense
We will never see guys sitting on the walls nor will we ever see those huge campaign walls (except on certain maps), however it would be nice to select a section of wall and add a Tower to it. When, like the Gate upgrade button, you push the ‘Bastion’ button, you get to add a Bastion/Tower structure to either side of the wall section. Obviously you want to put it on the outer part of the wall, but its up to you! (this covers the problem which would happen if the campaign walls could be built - the facing aesthetics of it). The tower addition would essentially be a more fitting Battery Tower (scenario building) that is part of the wall.
Wall Aesthetic
Wooden Palisades aside (as wood defenses would broadly the same), Stone Walls should be more fitting for the Northern/Western, Med and Central/Eastern European civs. North/Western civs should stick with the current stone, Mediterranean civ walls should be much lighter (almost beige/white) stone and Central/Eastern Euro civs could either be the current stone or red brick (which ever is more fitting). Towers could aesthetically be round and tiled or square depending on region.
Pioneers, Engineers & Sappers
Whilst artillery will be the best against fortifications as before, I would love to see Pioneer/Engineers for most civs, who are expensive but would be a multi-tool Siege Trooper! If they can get close enough to a wall, they can blow it up with a timed Powder Keg (costing resources to deploy it). Engineers, along with being able to construct Wooden Palisades even when Stone Walls are researched (linking to the first item), can also build:
Stockades
Making using of a Campaign only building, the Stockade is a cheap, garrisonable fort. Whilst having no firepower of its own, they can be garrisoned with any Gunpowder unit to provide musketry-fire as its attack, with attack power based on how many units garrisoned. As a cheap structure that can only be built by a Pioneer/Engineer, it won’t last very long again mortars and cannons, however could be useful as a forward base (for shipments) and as something to hide in if outnumbered.
Gambion
Gameplay-wise, the artillery equivalent to the Stockade. Gambions were wicker cylinder/barrels that were filled with earth and used to fortify cannon batteries. Once built, Gambions appear as a circular wall. When you garrison artillery into it, the guns appear in the middle of the ‘nest’, which add slight Artillery Resistance and extra range. The downside to this fortified battery is that the cannons have to be ungarrisoned then limbered up again, so they are sitting ducks when anything has gotten close.