Something I've been thinking about lately... So as we all know, users with transplanted eyes do not compare to the eyes' original owner in power and so on. Nagato with Madara's Rinnegan, while highly proficient, still couldn't do things Madara could with the eyes. Madara regaining both of his eyes was regarded as something to be feared, while Nagato had both and it wasn't noted to be a big deal. Obito could handle only one of Madara's eyes and the power and usage were far below that of both Nagato and Madara.
With the known and obvious out of the way, let's look not only at Madara, but Sasuke as well. So technically, Madara's eyes are not even his own, but Izuna's, while Sasuke's belonged to Itachi. So Madara's eyes are basically this:
- Izuna's cells
- Izuna's chakra, or at least Sharingan chakra
- Madara's own chakra or at least Sharingan chakra
- Indra's chakra
- Asura's chakra
Sasuke's eyes are:
- Itachi's cells
- Itachi's chakra or at least Sharingan chakra
- Sasuke's own chakra or at least Sharingan chakra
- Indra's chakra
- Asura's chakra
- Or both merged into one, simply Hagoromo's chakra
More complex than it seems at first isn't it... so with that out of the way, there are two things that I wonder about. First, If Izuna were to be alive and to take Madara's eyes, thus his own eyes back, would the 'Eternal' part of the Mangekyou be retained or would it revert back to Izuna's own Mangekyou Sharingan? Second, would Izuna be equally compatible and powerful with Madara's Rinnegan as is Madara or not, since the eyeballs are Izuna's own, thus, while the Rinnegan part of the eyes isn't his, he is still technically the eyes' true/real/original owner.
The Sasuke/Itachi case is the same as above, so no need to write it again.
/opinions?