Thinking out loud... What if HWPs were set to require a specific facility -- like aircraft require a hanger -- so each defense facility can store one HWP and none can be stored in general storage? They take up some space so that would free up some general storage, but more importantly it would give each defense facility a specific type of HWP it could store -- plasma tank only in plasma defense, fusion ball tank only in fusion ball defense, laser tank only in laser defense, and have the cannon and rocket tanks both able to be stored in missile defense facilities.
Thank you for the years/decades of work on XCOM!