True, but why just EMS? Unless Madara is the first person to actually discover how to mutate the eyes into Rinnegan. I can't throw the Elder Son's theory without seeing the story showing him in the mix. It's true what you said, the sons are directly related to the sage himself. But why can't he have 3T, MS, EMS, and Rinnegan? It would just feel that the Younger Son was the one to wield all his full power and the Elder Son is limited because he doesn't have the strongest one of all. That's why i suggest that if it's true that the Elder Son never had the Rinnegan. He must of had a complete different eye compare to Nagato, Madara, and Obito. Your suggestion of the EMS is a bit far-fetched because although i won't jump to conclusions. The only way to have substitute the idea of him not having the Rinnegan is for his EMS to have all of the Uchiha powers in one eye. Possibly powers that Madara doesn't know of.