As it is now, defenses are very straight forward plain attack and shields. A new type of defense with mechanics would add an interesting layer of complexity and gameplay diversity.
Fot this i have thought about the folowing:

Anti-Air Gatlin Gun Defense System:
"With the development of high tech weapons, the effectiveness of these has been seen do perform poorly on small fast moving targets due to their lack of pinpoint accuracy and sluggish targetting and positioning systems. After years of research scientista have come to realize that ancient balistic weapons could outperform the most modern piece of technology in some scenarios."

Attack strenght: 50
Shields: 0
Structural Integrity: 30,000

Costs:
Metal 25,000; Crystal 5,000; Deuterium 2,000.

Rapid fire:
Light Fighter: 100
Heavy Fighter: 80
Cruiser: 10

Ineffective vs most ships but capable of taking doen fast moving smal targets.

Comparable damage to heavy lasers vs cruisers. Plasmas vs lf.

Pros. Adds more complexity to defense rather than plain and flat stats. Does not invalidate any curreng defense.

Cons. Outshined by endgame defenses. Has rapid fire.