"Okay guys, the only Call of Cthulhu character sheets are these Dark Ages ones, so you'll have to write in Computer Use and such..."
"What, we're not actually playing Dark Age? That would be boss."
"Well, I was thinking about this thing with vikings-"
"We could be Templars! I love Templars! Do a Templars game!"
So eventually I'm going to run Call of Medieval Cthulhu, where the PCs are a group of Templar associates. You could have a knight, a monk-scholar, a sergeant-at-arms, a traveling merchant, etc. (Since the Templars owned so many properties and business enterprises across Europe, you could work in almost any sort of medieval character as someone who was working with/for the Templars, even if they aren't a knight/priest/monk.) Current plan is to set it just before King Philip IV outlaws the Knights Templar--it lets me potentially use the looming persecution of the Templars as campaign-ending plot element.
I also think I'm going to work in the idea that the Templars themselves are riddled with Mythos forces. Going with more of the secret-society/mystery-cult concept, where the higher you are in the organization, the more of the "mysteries" are revealed to you. And it's all couched in Christian mythology and medieval superstition--the PCs may even get to use magical artifacts or mythos magic in the form of what they believe are saint's reliquaries, fragments of the Cross, and Solomon's lost writings. There will totally be a Holy Grail and a Head of John the Baptist, and they're not actually going to have anything to do with Christianity. I think I'm going to make up (or leave ambiguous) the Mythos power behind the Order's corruption--just referring to any manifestations as angels and demons. So they might end up fighting Dagon or Shub-Niggurath, but as far as they're concerned they're taking orders from Garbriel or Metatron.
I'm planning it in terms of three approximate arcs. First, the "monster hunting," where the PCs are just sent to do seemingly-mundane jobs and end up brushing up against Deep Ones. Then comes the "holy warriors," where they start taking on increased powers from the leadership to handle more difficult and mysterious forces. And then the climax, where they have the twin Big Bads of the French monarchy and their own cultist brethren.
