Madaras younger brother!!

No really.

Why would taking his brothers eyes kill him?

It would simply make him blind, Obito didnt die once Rin gave kakashi his sharingan, he stayed alive til he got crushed.

Eye transplants dont kill the donor.

So Madara dies against the 1st, and his brother comes along and steals his eyes back, with Madaras lenses too, EMS!

A hint to this is that guys Byakagun!

Its symbolic, he has a donour byakagun to see through danzos hidden eye.

Its saying Byakagun is the key to tobis eye.

So, hyuuga clan, the brothers SWITCHED, (upper and branch family).

I believe Madara and his brother may have switched, so everyone who thought was Madara was actually his younger brother, so the younger brother is technically Madara.