This is basically a rehash of what I've said before in the CK1 forum a long while back.
I'm hoping that the Crusader army is automated, where it follows the quickest path to it's destination, so it can't be abused by the player (and as well being fair to the AI). During a call for the Crusades, or while it's currently active, you can pick characters you want to go, and what army you want to send with them (ie, send 5,000 soldiers, 12,000 soldiers, etc). While the Crusader army marches to it's destination it can *pick up* more soldiers by automatically turning some local population into soldiers.
If your characters are successful, they can get some *special* traits that are Crusader only, such as a nice healthy piety bonus, prestige, etc. If your characters and army do the bulk of the warfare, you'll get the land yourself. This is sort of, in an abstract way, similar to Sins of a Solar Empire's Pirate system, anyone play that? The more money bid on your enemy, the more likely the pirates will attack them. Indirectly, the same type of thing can be said about this type of Crusader system. The better characters you send, the bigger the army you send, may grant you special perks, such as getting the Crusade target under your direct control, very rare titles, etc. Therefore it may be more worth your while to send your heirs for those special perks, as well as contribute more forces to this venture. It's sort of like bidding, do you bid less and play it safe (less characters, less soldiers), thereby less likely to succeed in the Crusades, or bid more and risk a lot more (more characters, more soldiers), yet you may get more out of it in the end.
I would not like to see a player option for manually controlling a Crusader army. If people are wondering why, go play MTW2 which lets you manually control Crusading armies, and see how much abuse you can do with it - seriously it's pretty bad. They modeled the Crusades much better in MTW1, it was more of a natural disaster. If a Crusader army marches through your lands you have two options, either let them pass and let it absorb some of your people, or stop them by attacking it. It followed an automated path which usually marched straight to the objective (not player controlled, what-so-ever). This gives a Crusader army more of a natural whirlwind effect (ala MTW1), rather than feel like it's a bloated manually controlled army marching around ransacking non-crusading targets because it can due to shear numbers and nothing to stop it (ala MTW2).