my theory is that there are six powerful "clans" or powers that make up the sixth paths.
1st path-senju
2nd path-uchiha
3rd path rinnegan(nagato)
4th path- byakyugan
5th path- sage (natural energy/ y wouldnt the rakidou sage have access to this)
6th path- 10 tails power
Its pretty obvious that Madara: is from the Uchiha clan, took some senju from the 1st (like danzo), wants the rinnegan from nagato (third path or whatever), is after the bijuus and can probably open the celestial gates.
This is all just a guess tho. Im not sure if Hyuuga fits b/c (i think) the Sharingan came from the hyuuga's byakugan
I think these all are geared toward blood line limits......
Here's something interesting:
also, I think the battle is being set up like this:
madara has the 1st genes, maybe just his left side where the eye that closed was is the hashirimas (im guessing that is the eye that he replaces with his sharingan vault). Itachis gift to naruto is partial sharingan's ability so naruto would have sharingan, 9tails and sage making naruto just as close to rikudo as madara is.
Madara's plan:
1. Rinnegan
2. Sharingan
3. Byakugan
4. Hashirama Power
5. Uzumaki Power
6. 9 Bijuus
So....maybe things will fall out like this:
Madara and Naruto meet up over the 9 tails AND maybe madara comes after Hinata (heir to the hyuuga clan..... Itachi gave naruto some sharingan abilities or something to counter it. Sasuke fights naruto....naruto either wins or convinces Sasuke to help fight against Madara.
The two of them fight madara and sasuke gets injured....gives his eyes or eye to Naruto and Naruto finishes the fight.
Or maybe Naruto inherits the brakugan somehow... it would go with the path that he is on with his sage power and his sagey waves.