There is one mechanical ladder tied to 5e level, and a local title used in fiction. Promotion is earned in play: a commendation, a trial, a council vote, a wanted-poster notoriety, a temple's acknowledgment. Experience points alone should not automatically change how a Fire academy or an Air Council addresses you.
UNIFIED TIERS
Levels 1-4: Initiate
Levels 5-8: Adept
Levels 9-12: Master
Levels 13-16: Grandmaster
Levels 17-20: Legend
NATION TITLES (same tiers, different words)
Fire Nation: Recruit, Soldier, Lieutenant, Commander, Legend.
Earth Kingdom: Student or levy, Guard or disciple, Specialist, Master or official, Legend.
Water Tribes: Pupil, Warrior, Senior warrior or healer, Master, Legend.
Air Nomads: Student, Nomad, Master, Elder, Legend.
Avatar (if in play): Awakened (native air only), Traveler (second element), Realized (third element), Fully Realized (fourth element; Avatar State still risky), Legend (controlled State, past lives in concert).
SPECIALIST TRACKS use the same tiers with local names.
Yuyan: Novice, Marksman, Elite, Shadow Archer, Legend.
Kyoshi Warriors: Initiate, Warrior, Sister, Headwoman's circle, Legend.
Dai Li: Candidate, Agent, Senior agent, Under-secretary, Legend.
Fire Sages: Acolyte, Sage, High officiant, High Sage, Legend.
GOING ROGUE keeps your mechanical rank and changes the title: deserter, wanderer, outlaw, independent master, fallen sage. A Commander who defects is still dangerous. An Air Master who leaves a temple is still a Master when the wind rises.
AT THE TABLE: write both the unified tier and the local title on the sheet. When you address a canon or original official, use their world title (Fire Lord, Headwoman, Elder, Grand Secretariat), not 'level 16 NPC.' When a PC is promoted, make it a scene: a barracks reading, a council nod, a stolen chop, a rumor that has become a name.