I have two more suggestions based on my earlier comments about crewing.
1. Introduce crew pay
This might only apply to those in the pool for simplicity's sake. There is a certain charge per person, and a certain charge per grade point. This way, you can't build up a huge reserve of crew like you can now, at least not cost-free.
2. Reintroduce mothballing
I know that at one point, this was in the game, but it got removed for balance reasons. It should work fairly well with my other proposed changes. The problem with a vessel coming out of mothballs is that it has no crew, it's not worked up, and its maintainence clock should tick slowly (maybe 10 to 1 or so). So to put it in action, you have to get a crew, overhaul the ship, and work it up. While, presumably, you're in a war. And the crew is likely to be pretty green. Particularly if you have to pay inactive crew, you won't have a huge number sitting around waiting for the war. So a ship just coming out of mothballs has a green crew, a TF training of 0, and time on the maintainence clock, compared to one that was already in service. All in all, not a bad trade.