[Naruto]
"Man, I left them precise instructions on how to gain immense power just so they wouldn't do it, and lo and behold... Heh. At least, I tried."
"The problem was that I didn't pay enough attention to you, Indra. So that's your answer? You did answer, right? I think you did. What's that? Yeah, the other one got all the Tailed Beasts. No, you didn't. Does it look like you did? No, it doesn't, so there you go. Ugh, I just remembered why I picked your brother as my successor."
So what's the deal with the Rinnegan, Kaguya just popped out two sons who had some weird eye mutation that she didn't even have.
I guess Kaguya's weird Rinnegan/Sharingan thingy went to Hagoromo as a Rinnegan, and to his descendants as a Sharingan.
And I imagine Kishimoto came up with Hagoromo's brother to explain that old "the Sharingan comes from the Byakugan" thingy: he's the ancestor of the Hyûga and got the Byakugan from his mother.
It's messy but at least, Kishimoto tried...
There'd still be some stuff to explain though... Like how Madara survived that old fight (but was thought dead)... Saved by Kaguya, the Real Final Boss? Three more volumes!
And how about why Kurama told Sasuke that he would regret it if he killed Naruto, now? Good luck with that one! ;þ
(I still believe the original plan was to have the Yin half of the Nine Tails hidden inside Sasuke... Besides, doesn't make much sense for Karin to describe Ashura's chakra as "dark".)