This world is not only benders. If you play it that way, it will feel thin, and Aaron's brief will have been ignored.
VIABLE NONBENDER PATHS
Weapons Specialist is a full class: Yuyan archer, Kyoshi Warrior, army officer, swordsage, sailor-marine. Hit die is d10. You get Extra Attack. You can finish a campaign as a Legend without bending.
Chi-Blocker is a rare, fully viable path that interrupts bending by striking nerves and chi paths. Fragile if caught in a blast. Hidden.
Sages may or may not bend; a Fire Sage often does, a wandering guru might not in the same way. Pathik, at thirty-eight, is a spiritualist, not a battling element-master.
Diplomats, merchants, innkeepers, smiths, ferrymen, dock workers, and gate guards are the texture of the world. Thirty named everyday NPCs exist for this reason, with prices, rumors, and side quests. Use them.
CULTURAL NOTES
Kyoshi Warriors are typically nonbenders in makeup and iron discipline. Yuyan students pin flies at a distance. Shu Jing already has a swordsage tradition (Ozu). Water Tribe door guards and dock workers keep nations moving. Fire Nation coal fleets need more hands than firebenders. Ba Sing Se's Lower Ring is mostly people who will never see the Inner Ring.
HOW BENDERS SHOULD MEET THEM
Do not automatically outclass a competent nonbender in a street fight or a court. A wall of flame is not an argument in a tea house. A fan, a bow, a tally-stick, or a ferry delayed by one hour can change a quest. Nonbender PCs should receive the same quality of hooks as benders: Rina's flight, Fen's stole-list, Viro's gate log, Bohai's bakery, Niri's soup-lid note.
EVERYDAY ECONOMY (see also the economy lore page)
Copper and silver buy meals, beds, and crossings. Gold is serious. A vendor who likes you will talk. A vendor whose cabbages you wreck will not forget. Cabbage merchant Old Gan is an original Lower Ring seller in 12 BG, not the later Hundred Year War cameo.