The Spirit World is not a dungeon crawl with better lighting. It is another side of the same world, closer at solstices, at desecrated woods, at the oasis, at Roku's smoking island, and wherever dragons have been killed for sport.
THERE IS NO TRAINED AVATAR TO MEDIATE. That fact alone is already changing the weather of spirits. Healers say the oasis has been 'loud' since the night Roku died. Hunt-sites draw strange occurrences. Sozin has blamed visiting Water Tribe investigators for spirit activity that his hunts caused. A joint Water envoy may still be trying to learn the truth and being treated as a nuisance.
NAMED SPIRITS AND WHERE THEY ARE
Avatar Roku and Fang linger at a volcanic island echo in the Spirit World and, on thin nights, at the Spirit-echo ridge of the ruined island. Later, a statue and sanctuary on Crescent Island will become a place to speak with Roku. He is not a combat summon.
Ran and Shaw, the eternal dragons, roost in the Cave of the Masters in the hidden Sun Warrior city. They test; they do not perform. They may refuse anyone who smells of a fresh dragon kill.
Wan Shi Tong, He Who Knows Ten Thousand Things, sits in his library in the Si Wong Desert. Visitors must add knowledge, not only take it. He already has material on comet-cycles and human war plans, and he is in a bad mood about both. His fox-like knowledge seekers (including one called Dart in this world) report 'war tourists.'
Koh, the Face Stealer, lives in his lair in the old ocean spirit's realm. Show no emotion, or lose your face. He still has faces from Kuruk's era and would be interested in those who grieve Roku too openly in his realm. This is a set-piece, not a random encounter on a canal.
Hei Bai is the forest spirit of Senlin Village's sacred woods. Peaceful if the forest is whole; monstrous if it is harmed. Logging and 'resource' teams have started measuring the trees.
Tui and La, Moon and Ocean, swim in the Spirit Oasis of Agna Qel'a. They are not conversationalists. Violence here is a world-class crime. Do not bottle the oasis itself.
The Stone Dreamer, a huge many-legged canyon-crawler spirit from era sources, slumbers near the Eastern Air Temple. Miners woke it before. They may try again now that the Avatar is not here to mediate.
Lion turtles exist in the deep past and in hiding. Do not drop one in a capital. A rumor or a once-in-a-campaign sighting is enough. The Mother of Faces, in the Forgetful Valley, is a distant possible quest, not a neighbor.
DRAGON HUNTS
Sozin started encouraging dragon hunts earlier in his reign, even though he had reason to know they cause spiritual and environmental harm, to bind the nobility and recover popularity. A few dragons died at first; the sport grew. Killers take the honorary title 'Dragon' and a rumor of multiplied firebending. Dragons have become more aggressive. Spirits gather at death sites. This is already a wound, years before the comet.
OPTIONAL DARKER THREAD (original, not hidden canon): with no trained Avatar, a greater spirit hunger may gather — Koh becomes bolder, hunt-site spirits merge, or something older tests the borders of the world. This is not Vaatu walking in unannounced. Use it only if the table wants a spiritual doomsday beside the comet.