Adzi

The Call of Cthulhu system can be used for plenty of stuff besides Lovecraftian horror. You could do a whole gangster or spy episode for instance (probably not using the Sanity rules a lot though). It's a robust system, which I inherited from Steve Perrin's BRP, and which Mike Mason & Paul Frick have overhauled for 7th edition. I take no credit for the basic system, only for adapting it to the modern age and horror. And of course adding all the monsters, spells, Sanity rules, etc. But one thing that kind of frustrated me back at Chaosium is that periodically I wanted to use Call of Cthulhu for a horror adventure that DIDN'T involve the Mythos. Like with a ghost or a werewolf or an army of zombies something. I was occasionally able to squeeze one in. For instance, in Masks of Nyarlathotep, originally the London segment was centered on a werewolf (Larry DiTillio wanted an homage to The Wolfman) but Lynn Willis made us add the Cult of the Bloody Tongue so it "connected" better to the rest of the campaign. I'm not saying he was wrong, but this happened again and again. Now while Call of Cthulhu CAN be used to play a non-horror adventure, I don't ever do this myself, because the way I see it the players have signed up for a night of horror, intrigue, and revelation. Not Liam Neeson in "Taken". But if your players are up for it, go right ahead. As I said, the game system works perfectly well for non-horror concepts. We even had rules in the very first edition for creatures like pythons, leopards, and so forth. And werewolves! Who else has played a game of Call of Cthulhu without the Mythos, or at least with only a trace of it?

Players & keeper can make any agreement they please. What is bad is DURING the game for players to try negotiate rules adjustments - slowing down play and turning what should be decisions into wheedling. Once the game has started, for the good of the game, the keeper should be the primary interpreter of the rules.