I think we all know the Tengu theory.
It's a great theory, but nothing more.
The tengu stuff that is in the manga comes from the jap mythology, just like the kyuubi and slug,snake, frog stuff. Kishi is just using it in it's own way.
The problem I have is that the theory is too much about the uchiha clan. It outshines Naruto too much.
And it's a bit late to start something like this. We are at volume 37 and there have only been small hints.
I think the manga had to go for quite some time to include this theory.
It's okay if it turns out to be true, but right now it's only a theory.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"