I would think that the idea of suzerainty should be in the game, which means that certain kings had claims to overlordship over other kings. This could be because of bonds of blood, proximity, or some other historical reason. So in Drachenfire's example, England's claims to domination over Ireland, this involved Henry II's (and later John I's) claims of suzerainty, which had been confirmed by the English-born Pope Adrian IV, in order to bring the independent-spirited Celtic Church into the Roman fold. (Even in this case the pope refused to grant the Plantagenets the title "King of Ireland," which would have gone to John Lackland, at that time his father's favorite son.) This might work similar to sphere-of-influence in EU3, but it would be more complicated than simply drawing up the proper papers.
Maybe papal approval (as in England's case) and the approval of the other powerful neighbors would be the case. (In the case of Ireland, the King of Scots invaded to create a second front in his wars with that of England; and earlier France during the turn of the XIII century urged the Pope to guarantee Scottish independence; the Pope by this time was becoming the ultimate arbitrator over who should dominate whom.)
I submit these rather scattered ideas, but I hope that my point is clear; I would rather see a system consisting of claims worked diplomatically (and backed up by force of arms) rather than CK1's system of "I have 5000 prestige from my conquest of the Holy Land. I now lay claim to every barony in Ireland." Without badboy to ruin your relations if you go on a conquest spree, there need to be IMHO more stringent means of establishing claim to this territory or that.
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