EDIT AND SUCH:
Well, due to the "friendly" feedback on the no turrets idea, I have a different suggestion and the reason why I dislike turrets: 
Back in GC, the biggest problem in the Deathstar map was that the imps would only use the turbo laser turrets cause it was easier to shoot down the rebels like that. If you ask me, it was way boring for the rebels cause you were basically powerless against them.

Other idea: how bout the ISD would only have huge, slow firing, VERY slow projectile turbolasers, that would be so nerfed up that you could only hit big'n slow stuff like blockade runners incase you can get those steerable. (Can't you script your own vehicle types?) So to take out the small craft the ties would be needed, and to take out the huge stuff turbolasers would be PERFECT.