I'm not a Fate buff but seems like all entries on the franchise are centered on getting the Holy Grail and getting that wish. There are those who would attempt to put an end to it though.

So I thought, why not introduce in our RP a faction specifically made to destroy the Holy Grail and those who would attempt to summon it as something new and (kind of) original. Could be a government-sponsored team or an NGO (I'm thinking G.I. Joe-like but that's only me). They could try to recruit the players to aid their cause.