some_writer
I don't know how you would use a piece of Raava, but in one fanfiction about Zuko, he learned the sub-elements of firebending by copying the styles of Other Nations. Like Lightning, fire, ash control or something like that, and lavabending. You can make it so that he first tries to learn the sub-element, and only then the full-fledged one. Or something similar.
My recomendation for you is to find a interesting way to present Kai to the elemental world and now because he is young, remember that the avatars are a huge deal, if the world realize that suddenly there is a Second Avatar in the fire Nation then everyone will go crazy, criminals, politicians, goverments, the white lotus will take him and teach him how to mantain balance while they find out why they are two avatars now, you can take this path now that mc is young so people have time to adjust to the idea of having two avatars instead of 1. Also it gives mc access to korra