Here, a quick mock-up:
Engineer Class:
Squaddie:
*Deploy Turret - Throw out a Turret with one shot of automatic Overwatch; 2 Turrets per mission, 2 turn cooldown
Corporal:
*Precise Targeting - Confer a 50% chance of Critical on Overwatch shots taken by a turret.
*Shield Defense - Deployed turrets are mounted on a pylon that provides full cover.
Sergeant:
*Malfunction - +20 Aim and +20% Critical chance against Robotic enemies.
*Low to the Ground - Movement does not trigger enemy Overwatch.
Lieutenant:
*Improved Gadgetry - Arc Throwers have a wider radius and a 10% increase in their success rate.
*Repair Expertise - Engineer can repair SHIVs in battle without an Arc Thrower.
Captain:
*Auto Reload - Each Turret has two shots in Overwatch instead of one.
*Proximity Bomb - Turrets are armed with proximity sensors, causing them to explode if aliens get within the bomb radius.
Major:
*Turret Portability - Can carry one additional Turret into battle.
Colonel:
*Quick Hands - Deploying a Turret doesn't cost an action.
*Remote Control - Turrets do not deploy in Overwatch and can be manually aimed.
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Add in Laser Turrets and Plasma Turrets as Research/Foundry options and bam, you've got a fifth class.
The framework's all there. You're welcome Firaxis, just don't forget to thank me in the credits.