The rules of the Spirit World are a little vague, to be sure. Way back in Book One, Hei Bai the Black and White Spirit lead Aang into the Spirit World for the first time where he met Fang and we got some quick world-building rules about the Solstice and the Spirit World and whatnot. 

My initial thought is that Wan and his entry into the Spirit World during Harmonic Convergence is a human physically entering the Spirit World through one of two portals that are sealed for all time after the battle. This means that no one else, in theory, entered the Spirit World in this way, which could imply that Bending totally works in the Spirit World if you can get the entirety of your being into the realm. Aang was carried into or meditated into the Spirit World instead of physically being transported there.

Then again, the Avatar’s past lives are given ghostly presence in the Spirit World despite other humans not getting the privilege and Iroh managed to see Aang on a Spirit Fang, giving rise to the backstory that Iroh had attempted to enter the Spirit World to save his son.

And then there’s Koh’s tree looking like Vaatu’s tree even though Koh’s counterpart is the Mother of Faces. And there’s Aye-Aye acting somewhat similar to the Mark Hamill Monkey outside Koh’s tree.

And there’s the Spirit’s immortality in the Spirit World versus mortality when taking on a form in our Realm.

There are a lot of rules, and I don’t think we have all the information yet.

-Da7e-