One thing I don't much like is not having an in-character excuse to do something. 10-turn forced leave falls under this category. "Oh, the French are marching on Frankfurt with a giant army, but our greatest general was in the field only 8 turns ago! Let's entrust the defense to this inexperienced general instead." Doesn't make sense.

Of course, we just need to develop proper excuses. In case of an invasion, the leave rules should not be in effect. In fact, we shouldn't call it "leave" at all, but simply a reassignment to a reserve command. I don't think this should be a fixed 10 turns active, 10 turns reserve rule, but instead that rule should be followed only if there are enough generals in the active command. The chancellor should be able to activate reserve generals when he needs more active generals. But, under normal circumstances, he should be encouraged to rotate generals. We may run into a situation where we don't actually have enough generals, just as easily as the opposite. The rules should remain flexible.

My suggestions, made in the TWOS post-mortem thread, focused more on permanently getting older generals with many battles out of the way if there are young generals without commands. But, instead of forcing them to go on leave (a bad thing), they can be given even more important commands (a good thing). It really comes down to a slight decentralization of the army structure. This particularly applies as the Empire gets big. The army as a whole could be divided into a few commands, which could be given to older generals. Those generals would not fight battles, but only give specific orders to subordinate generals. This is what I'm thinking:

Back when Servius became Consul, and the Seleucids invaded Thrace, I believe it was Cornelius Saturnius who sat out of that fight because much of his army had been used to reinforce mine. We only had enough troops for two armies. What could have happened instead is that I could have been assigned to be the Commander of all the Army in the East, or something equally fancy that really means that it would be my character sitting in the town nearby, while the two younger generals command the armies. I expect that Servius would give me an order like "Expel the Seleucids from Thrace, you have this and that unit under your command." I would then take a good look at the map, come up with a super great plan, and then give the orders to the armies, which really meant sending orders to Servius, who would make the little men on the map dance as I said, and pass around savegames. That way, the two younger generals get to fight the battles, and I get all the glory.

So, my point is that there can be a structure of rank beyond a single-stack commander. There should, of course, be a limited number of these, and I suspect that the Senate should be the one to appoint generals to those positions. That only need happen once the starting generals start to get too experienced, and if younger generals are lacking field assignments.