If the sage's yin+eyes=the sharingan

[rollin]

The Older son didn't inherit the yang chakra making him most likely have an ems with the first distortion of the sage's eyes making it swirl
ems is the last power up before rinnegan
when you implant an ms you also get yin chakra
susano'o resembles genjutsu chakra

genjtusu is yin element

sasuke felt itachi's power flow through him and the susano'o both look the same

it also makes since for naruto and sasuke
sasuke's powerups:ms(yin chakra)+ms(yin chakra)=the yin chakra of the older son and eyes
Naruto:uzumaki's already have life-force and yang chakra+kyuubi's yang chakra=the same flame-like chakra of the younger son
so if the older son were to inherit the eyes and the yang chakra instead of yin chakra what would you think the eyes would be like
is that what the byakugan is
sharingan uses genjutsu techniques and
byakugan makes you able to do chakra based techniques

[Big bengi]

I had a theory pleading about this because the senju are mostly about taijutsu but am not sure.

[Izuna Kakashi Senju]

good theory.