As such, I was thinking of next tier walls, possible recipes for them
Tier 2: Steel Reinforced Wall
Recipe: 5 Wall + 2 Steel Plate + 10 Refined Concrete
Requires tech: Stone Wall + Gate + Concrete + Steel Processing + Steel Reinforced Wall (Self)
Health: 700
Tier 2 Gate: Steel Reinforced Gate
Recipe: 1 Steel Reinforced Wall + 10 Steel Plate + 2 Engine unit
Requires tech: Stone Wall + Gate + Concrete + Steel Processing + Steel Reinforced Wall (Self)
Health: 700
Tier 3: Energy Shielded Walls
Recipe: 20 Steel Reinforced Wall + 10 Energy shield + Polymer Blend (Liquid)
Requires tech: Steel Reinforced Wall + Coal Liquefaction + Energy Shield + Energy Shielded Walls
Health 1500 (Unpowered) + 500 Shield hitpoints when powered by an energy source
Energy consumption: 1 MW when active
Drain: 24 kW
Recharges 12 shield HP per second
Only activates and consumes energy when enemies are nearby (similar to the turrets activating by themselves)
Tier 3 Gate: Energy Shielded Gates
Recipe: 1 Energy Shielded Walls + 2 Processing unit + 4 Electric engine unit
Requires tech: Steel Reinforced Wall + Coal Liquefaction + Energy Shield + Energy Shielded Walls
Health 1500 (Unpowered) + 500 Shield hitpoints when powered by an energy source
Energy consumption: 1 MW when active
Drain: 20 kW
Recharges 12 shield HP per second
Only activates and consumes energy when enemies are nearby (similar to the turrets activating by themselves)

(Image from: "Sci Fi Wall" on UE Market Place, by Vexejer, just to illustrate)
Liquid: Polymer Blend
Made at: Chemical plant
Requires tech: Coal Liquefaction
Recipe: 20 Crude Oil + 40 Coal + 10 Plastic Bar
Values can definitely be tweaked and recipes can be changed or balanced, but I really think the game would benefit from having better, but more resource expensive walls against late game strong bitter hordes that can simply obliterate the early game walls.
The Polymer Blend could also be used for other future recipes, such as "nanotubes" or other space age technology and the technology suggested is definitely within the scope of the games' scenario and the energy consumptions and health of the walls was based off of the items used in the recipe, currently existing walls, laser turret energy consumption, late game enemies, vehicles and the damage of late game weapons and enemies.