As was brought up in the swapping units thread and something I've been kicking around in my head some: Mothballing of units.
Let me start by saying that I understand that you don't want huge stockpiles built up, and I agree! However, I also think that we should have to just destroy units because we're not using them or don't have somewhere to immediately house them. Therefore, I propose:
Units may be placed into reserve at a storage facility. Units placed in reserve cannot be used in combat or as immediate combat reserves. Units in reserve cost 2% of their build cost, per month, to maintain. Placing a unit into reserve or taking them out of reserve costs 5% of their build cost. They take 1/2 their build time to be pulled out of reserves and made active. Units in reserve cannot be activated while there is combat at a location; those who would activate them are busy elsewhere.
These rules would also allow units that you can't pay maintenance cost for to be placed into reserve, instead of destroyed. The storage facility above could be an actual ground/orbital item to be built or not. I think the cost for transiting into/out of reserve and the upkeep cost of keeping them in reserve will discourage abuse while providing flexibility.
Use cases I see: I built a Venator, but I didn't have the cash on hand to upgrade the fighters immediately. I didn't want to wait another month before starting the build. Next month comes along, I'm ready to upgrade my Venator's fighters. I use my starfighter plant to built two hornet squadrons to replace 2 Z-95 squadrons. Without this, the Z-95s would just be lost; I have nowhere to put them/use them. With this, I can pay 6MC and place them into reserve status. Next month, I have to pay 2.4MC to cover their upkeep cost. Later on, if I lose a squadron in combat, I can (after combat is done, if it was at that location) pay 6MC to bring them back from reserve in 1.5 days and put them on a ship. Let's say that only lasted a month, so I paid 14.4 MC to store them, and it took 1.5 days to bring them out of reserves. In the end, I actually paid 74.4MC for that squadron, and an extra 1.5 days of time. However, that beats throwing away 60MC worth of fighters and then spending another 60MC and 3 Days to build replacements.
Something to discuss.