The Fire Sages are a canon order: keepers of ritual fire, calendars, funerals, and, historically, a duty to the Avatar as well as to the Fire Lord. In later decades many sages will serve the crown first. In 12 BG that future is not complete. This decade is when it can be forced, resisted, or delayed.
KAJA AND THE TEMPLE
Crescent Island holds the Fire Temple. Decades ago, a young Roku forced the Avatar State here and nearly destroyed the temple. Fire Sage Kaja pulled him out. Kaja is now very old, frail, exact, and heartbroken. He keeps the Sanctuary Antechamber and the Avatar Sanctuary doors for the next Avatar. A statue and shrine to Roku will grow here; the doors already matter. Kaja tests visitors, teaches the respectful, and turns away soldiers who will not bow to the Avatar's memory. The damaged upper temple, scar of that old accident, is a place for acolyte PCs and investigators. Lieutenant Azane's relief cutter already ties at the docks with manacles under the rice.
ONARU AND THE CAPITAL TEMPLE
High Sage Onaru holds the Hall of the Eternal Flame in the Great Fire Sage Temple in the Capital. He is political, sonorous, and afraid. He wants to preserve the order. Hesun has already asked him for a draft decree of allegiance to the Fire Lord "in emergency." He has not signed. Publicly he is helpful. Privately he may hire discreet PCs to learn what happened on the island. If the clock advances into years 2-5 without intervention, some sages bend the knee, and Onaru signs or is replaced.
RUGA AND THE SKY
Sage Ruga works in the Royal Observatory's comet-chart loft. He has already dated the next sky-fire to twelve years. Hesun has classified the charts. The Comet Charts quest is how a party makes the countdown visible: steal, copy, or persuade, then decide who in the world is allowed to know. A leaked date can unify enemies of the Mandate or accelerate first-strike planning.
WHAT SAGES STILL ARE
Ritualists, astronomers, funeral officers, and possible teachers of a new Avatar's fire — a role Sozin would love to control. They are not automatically wise. They are not automatically corrupt. A Fire Sage PC uses the Sage class or a Firebender on the Fire Sage track, with @Sage's Vigil, and should feel the split loyalty as a live choice, not as flavor text. The Dragonbone Catacombs under the Capital, restricted, hold royal dead and banned scrolls; Mandate or sages may go there by adventure only.
CRESCENT ISLAND AS A STARTING HOME
A sage, acolyte, or investigator can begin here the week of the funeral orders. The official story will arrive by cutter. Witnesses may arrive by the same cutter, not freely. Kaja's doors are one of the few Fire places still explicitly kept "for the next Avatar." That sentence is a hope and a target.