If I disbanded the magic army, then Norway would start out in a deep hole. I figured that Norway was intended to be able to conquer something, so instead of England I'm steadily picking up Finland, Karelia, and little chunks of Sweden when my chancellor is able to commit a successful forgery.
When the magic 10,000 are gone, I won't own London or Westminster Abbey. Instead I'll own the wastelands of northern Scandinavia, populated with nothing but reindeer and angry pagans. It's not like I came out ahead on the deal.
I'm surprised that Paradox didn't come up with some more creative game mechanic to simulate the three-way fight for England. It's the cornerstone event that starts the timeline.
Maybe we can think of a fix for an upcoming patch. Here's my starting idea: Under the right conditions, including a strong claim on a big kingdom, the king can get an extra-big long-term levy from every holding in the kingdom. The catch is that the king promises to grant his vassals major holdings in the target kingdom. The king is effectively the promoter of a business venture, and the vassals are the co-owners. If the invasion fails, then the vassals will be angry. If the invasion succeeds and the king doesn't grant the holdings as promised, then the vassals will be furious.
The king can only propose this sort of invasion if there are enough holdings in the claimed area to double all his vassals' holdings. He can propose it like a law, and then he has to wait for buy-in from his vassals. If the vassals agree to the special levy, then they get cranky if the king doesn't seem to be using it for the promised purpose. Of if he seems to be losing.
I'm no expert on medieval history, but this is my understanding of how the real William put his army together before Hastings. After the battle, William wasn't handing out feudal titles for increased managerial efficiency. He was paying back his vassals for having borne the cost and risk of supporting the invasion.
This game mechanic would push against the current strategy of keeping vassals small and weak. The only way to get these special mega-levies is for the king to make his vassals more powerful, which will help the game produce results that better track history.
An expanded version of this game mechanic could have the king negotiating for the mega-levy with individual vassals. If the king needs a defensive mega-levy and there are no new holdings to promise the vassals, then the vassals could make additional demands, such as a reduction in taxes or royal authority. This could lead to an event vaguely like the Magna Carta, where the barons push back and the king gets weaker.
But I'm just talking off the cuff here. I just started playing this game, so I gladly defer to more experienced players on how such a new game mechanic might work. I just want to point out, as a new player, that these 1066 magic mega-armies devastate my suspension of disbelief.
Historical note: William did get lots of armies from Flanders, Brittany and mercenaries IIRC.
While i basically like your idea, i'd suggest it to work like the ideas for a crusade, that provinces after an invasion (Norway fought a regular claim war contrary to William AFAIK) are not given directly to your Dukes and Counts,
neither to their direct heirs, but second sons or some dynasty members. Or somehow along the ideas going with crusades. I doubt very much though that Pdx will change that much in CK2. Maybe, but who knows.