Basically, I'd like to see a new combat mechanic, called Retire, that acts like Retreat, but places the ship back into the available Deployment pool. Or it could just be a change to Retreat; either way works. I don't think this can be done with mod code atm, otherwise I'd probably try it out.
Why?
Well, sometimes it would be really beneficial to remove fleet elements from the fighting and replace them with another type without losing them from the battle permanently.
For example, fighter wings; we may want to use them early until they've lost some CR but are still viable and then Retire them, holding them for use later, along with the carrier(s). It'd be a big improvement tactically, especially from the Admiral AI's POV, since it could engage in layered tactical approaches more easily when fielding big fleets. The system right now resembles a paper-rock-scissors game, where the wrong initial response may be all it takes to lose a scenario; it might be better to have a more flexible way to handle things.