Technically wrote about this once before but thought I could rework it.
Humans taking on the traits of spirits and becoming hybrids is something that has been seen several times in the series.
So far, most spirit/human hybrids (except for the Avatar and a few others) look uneven in a manner of speaking. Some parts are spirit and others are human, it looks bizarre and is kind of more disfiguring. However, what if there were a way to even out the spiritual mutations. In other words, make it look smoother and fuller, mixing the spirit and human traits more.
Currently, most people in the world fear getting mutated by a spirit but it is not out of the realm of possibility that some could become drawn to this. It could have combat potential or just be some kind of fashion statement. Spirit mutations could give fighters a decisive advantage and other people could also use them simply as a style/vogue.
Had the idea for this after thinking of animal/human hybrids from several other kinds of media. Could this be something viewers would like to see in the series?