The Gorosei are a council of the five highest-ranking Celestial Dragons: the “Highest Authorities of the World Government”. They are believed to hold the greatest authority over the world at large, although they are secretly subordinate to the government’s shadow ruler Imu, who occupies the Empty Throne.
The Gorosei seem to be immortal, and the most obvious explanation for that immortality is the Op-Op Fruit. Maybe Imu has immortality granted to any Celestial Dragon who shows sufficient strength, loyalty, and evil ideological purity. So the group grows over time, albeit very slowly.
If so, then the current Celestial Dragons are essentially auditioning for the role of Gorosei whether they know it or not. And there’s at least one Celestial Dragon who seems worthy right now: Figarland Garling.
Figarland Garling is strong enough to make a mark on a battlefield that featured Roger, Rocks, Garp, and all of their super-powerful subordinates. And we know he’s 100% committed to the monstrous ideology of his people. His current role as leader of the Holy Knights might well be Gorosei probation; I wouldn’t be surprised if the existing five all held that job at one point.
So maybe, when the Navy tried to buy the Op-Op Fruit 13 years ago, it was to sacrifice a slave doctor and make Figarland Garling into another Elder Star. Which would mean that, when the Donquixote brothers interfered and the fruit ended up in Law, Garling lost his shot at immortality.
If so, Garling probably holds a grudge against Law. And he might imagine that he can finally secure immortality by subjecting a defeated Law to some combination of threats and torture. That would set Law and Garling up to fight in the final war, which would make a lot of sense. Given Law’s strength, D initial, and story importance, we all know he’ll have a part to play in that storyline.
Tl, dr: Holy Knights get get promoted to Gorosei via the Op-Op Fruit’s immortality surgery, Garling lost his chance due to Corazon, he’ll try and get revenge / another chance during the final war.
Theory by sanctaphrax (https://www.reddit.com/user/sanctaphrax)