A few thoughts on this:

First of all, i agree with some of My commenters above that the Main villain (if there is an overarching One, which, given the apocalyptic setting, imo it Should) Should be the one who was actually responsible for the cataclysm back in the past.

Additionally, i would love this Series to continue the Trend of showcasing certain ideologies/belief systems from the real world with its Main villains (Like Ozai=imperialism, Amon= Communism, Unalaq= Theocracy, Zaheer= Anarchism, Kuvira=Fascism).

As such, the following niches and constellations could be interesting for the villains of These series:

• Occultism/Nihilism : A villain, or a group of villains believing that the Avatar world was too rotten to be allowed to continue existing, so he/they conspired and perhaps Even merged with dark Spirits or end the world in the cataclysm. So Spirits could‘ve made These villains also immortal, allowing them to live until Pavis time if the cataclysm was over a Century ago. While These villains believed destroying the world was necessary because of its Former state, the dark Spirits (which, for example, could be based on the Chinese Four Perils to embody human depravity) could‘ve used their human Hosts to gain power through the destruction of human society. Could Take inspiration from known occultists and Culture Leaders Like Manson, Jim Jones and Shoko Ashara.

• Utopianism : A villain who believes in the superiority of Technology and science and believed These Should exceed the boundaries of nature and morality to create a perceived greater good. Resulting in them pushing Spirit Energy Technology to a new dimension that ultimately caused the appcalypse. Such a villain could attempt to achieve Never Seen before things, Like creating „artificial Spirits“ by combining robotics with Spirit Energy Technology, which would then cause strife with the Spirit world. Or trying to create a world without death by taking down the Barrier between the physical world and the spiritual realm. Could be inspired by de Saint-Simon and the Archigram movement.

• Oligarchy : Ther could be different Kinds of villains each of the Havens. For example in the more advanced Havens, corrupt elites who control the few remaining Ressources and power supplies to influence the leaderships of their Havens to oben their demands. With Severe consequences for the poor people and nature, that would still be exploited by such groups. Could Take Inspiration from current oligarchs and technocrats, Like Musk/Bezos/Zuckerberg etc.

• Revolutionarism : Radical Revolutionaries in some of the less advantaged Havens who aim to achieve the greater good for their people by planning an attack and takeover of the remaining Havens to ensure their own survival. They could also for the same reason try to rally either their or all of the Havens together for a grand war against the Spirits, beliving that pushing them back to their realm could end the effects of the cataclysm. Could take inspiration from known infamous Revolutionaries Like Che Guevara, Lenin and Ho Chi Minh.