At the very least, I'd say that there needs to be something that will create an independent Jerusalem versus Jerusalem ending up a part of some king's or emperor's personal demense.
This is just an idea, but I feel like you could have some kind of lottery system based off of participation score to hand out all of the titles. Let the character who gets the most score decide by event if they want the Kingdom title to go themselves (abdicate current title), family, a holy order of choice, or no one at all (large amount of piety by saying it belongs to God and perhaps the Zealous trait). Then let everyone else going down the list of participants make the same choice on the remaining duchies, with the option of being independent replacing the last option.
Some other things I could see is that for any crusade targeting Byzantine de jure let there be an event that either returns the land to the Emperor, forms the Latin Empire with the Pope's blessing, or forms the Latin Empire and gain gold at the cost of razed holdings, excommunication, and a few other maluses. Also, how about a rare random event for any commander to find a relic such as the Holy Lance, Holy Grail, Crown of Thorns, etc. on any Crusade? Each could confer a stat bonus and a decent chunk of piety. Could also see an alternate version of the "Conscript Merchant Ships" decision that only appears during a Crusade- it would cost more but spawn more ships. Finally, I could easily imagine having either the equivalent of tribal armies become accessible during any type of Crusade, though instead of prestige have them cost piety or have small, cheap merc bands that cost a bit of piety.
On the flip side for the Muslims, let them have a decision to form Rum, find their own faith's relics, and perhaps add in a event that grants piety to whoever retakes Jerusalem. I could also see a decision to restore/add-on to the Dome of the Rock following the reconquest as Saladin did for even more piety, but that might be a bit much since the Muslims usually retake Jerusalem with ease.