I think to be a path you need to master one of the bloodline limits inherited from the Sage, that way not everybody with a sharingan would've been a path, neither would everyone of Senju - i.e Tsunade and her little brother - be paths. So 1) Madara became a path when he obtained the EMS, the pinnacle of the sharingan. 2) Hashirama became a path when he obtained the pinnacle of the unknown Senju/Uzumaki kekkai genkai. 3) Nagato reached the pinnacle of the rinnegan at some point, although what this point is is unknown, perhaps the ability to revive the dead. 4) Sasuke is/will be a path when his EMS is settled. 5) Naruto is/will be a path when he reaches the pinnacle of the Uzumaki/Senju kekkai genkai. 6) Unknown character, unlikely to be another EMS, or a rinnegan, can't imagine it being a byakugan user even though theoretically if it's descended from the Sage's eyes it could be. I think Jiraiya knew about the kekkai genkai of the Senju/Uzumaki, but using/awakening it is very difficult/dangerous, it probably requires tailed beast quantities of chakra to awaken, which means you need to have mastered a tailed beast first to obtain it, Hashirama controlled tailed beasts with his mokuton. Remember when that toad gave Naruto the seal key, he said it would help him master 'that jutsu', I think 'that jutsu' is either the Uzumaki/Senju kekkai genkai or it unlocks it. Therefore the final path, if it isn't someone with the Sage's eyes, is someone with the younger brothers kekkai genkai, so the final path will need to be capable of controlling a tailed beast somehow, not neccesarily as a jinchuuriki either. I've always thought mokuton was a pretty nifty technique to have, but it never seemed in the same league as EMS, what's wood gonna do against eternal flame but burn. If Hashirama were to beat Madara he'd have needed something on par with EMS, an equal/opposite counterpart, and Naruto's gonna need something similar too, cos even controlling the kyuubi alone doesn't compare with EMS imo.