I think it would be interesting if Avatar had some kind of spirit of death/Grim Reaper? They don't actually kill people, but take them to one of many parts of Spirit World (or to the spot where they wait to be reincarnated) after they pass on from the material world.
I guess that would be interesting. If they further more into the Spirit World, I think such spirits would appear, they’re so common beings in fiction anyway (although I like the Spirit World to kept as mysterious and holy as it is now). ATLA/LoK don’t focus so much on spirits as much as real life isuses anyway, which I appreciate better.