OK, so I spend most of my life living outside of the box and coloring outside of the lines. Zombies are a classic archetype. A lot has been done with the mythos and in some of the interviews I have seen with Romero (I have not watched the one on this site, yet), Max Brooks, the creator of the Walking Dead graphic novel (I cannot remember his name) all say that a lot has been done and they are looking for something new to move things in diferent directions. So, here is a crazy idea that I do not think has been done.
Knights Templars vs. Zombies.
OK, so know that you have caught your breath from laughing so hard, let me give you some backstory. One of the main charges against the KT was that they worship a talking head. A talking head has to be animated, but it really can not talk because it has not air pressure from the lungs, not to mention that depending on the exact point of decapitation, it may not even have vocal cords. So, we twist the myth. Accusers have it slightly wrong. The head does not talk, but it is animated. It is not worshipped, but it is kept safe and only shown to higher initiates as proof of the threat.
It is easy to see that if the dead return to life, it would thought to be magic and christianity could easily believe the cause is Satanic. So, it is much of a threat that the Pope would create a holy order to deal with it. Or, if not create, annoint an existing order with the responsibility. This can explain oh they become a personal prelature of the Pope. It can give the KT inital funding a recruiting.
The Pope may also want to keep this quiet because if people think that there are zombies, maybe Jesus Christ was a zombie and not the Son of God. THis could destabilize the faith. Later, KT can self fund and may slightly deviate from its inital cause. (Um, so if there are no zombies to fight, what the hell do we do now?)
Actions lead to increased violence in the crusades. The Muslims that rule most of that area may not have an understanding fo the zombie threat. If they were to find everyone in a town or city killed by the KT, that will really piss them off. If the KTs say, But, they were zombies! I can see the Arabs going, yea, right! Violence could quickly escalate out of control before cooler heads at higher level of government discuss things and present the head as proof.
Many zombie films are modern firearm flicks that are civilization destroying. Instead, this is mainly melee-based combat where the protagonists are hunter/killers rather than refugees in survival mode. I am sure that if I read the KT and crusade wiki enteries, I could blend some more ideas.