What would you all think about an "engineer" special forces unit? Essentially, they'd be considered special forces (perhaps this could merely be an extra research for special forces rather than a whole different unit). And they'd have the ability to sabotage buildings, and cause a certain amount of damage per amount of time. However, while they would be stealthy, once you ordered them to attack a city, they'd lose their stealth and you'd find yourself at war. But essentially, their use would be that you could disable an enemy's buildings, allowing you to delay their ability to produce resources. Perhaps, if stealth is overpowered for this unit, they could have air assault instead?

Do they declare when you use them, or no? Also, can you move them from city to city and stuff, or once you hire one in a city does it stay there?