This would also make them much more valuable - atm you can build one in ~ 10 seconds, so it feels actually more like a REALLY cheap uber-defense turret.
Actually.. for the fortress upgrades theres so many ideas coming to mind, hangar/drone bays. More Storage, in-storage-repair, dedicated repair range in several stages, several different armor layers, several different shield generators, and guns.. well a trillion different guns but most importantly..
super-charging-up-instant-beam-of-doom (like beam frigates?, just way more awesome) ^^