What relevance does he have in this? Naruto doesn't represent him at all, as he's not a Senju. He's been related to Rikudou the umpteenth times more than one can even remember, without any evidence to relate him to the Younger Son. I say Rikudou and the Elder Son because those are the ideals that are shaping the final fight. The relation to the Bijuu's is seen only by two people in the entire Naruto universe; Hagoromo and Naruto. The need for radical change and it being forced is done through power and represented through Sasuke and parallels the Elder Son's views. Technically, by extension, Hagoromo did choose the Younger Son's ideals to succeed him so he is inherently part of this equation, but this is becoming far off topic from your initial qualm about the Dojutsu users. Yin and Yang is not relegated to the sons solely. While they are part of it, and have been represented by Madara and Hashirama already, the Yin and Yang concept is prevalent entirely in the manga beyond them. The darkness (Yin) of Konoha and the light of Konoha beyond that (Yang) which was done primarily by Danzo an Hiruzen is another representation on a grand scale of this concept. It isn't solely about the sons.