Neji's death is similar to the state of naruto and sasuke before hagaromo past on his power's to naruto and sasuke. My theory is that hamura will give neji his transmigrant re-encarnation or whatever you want to call it power to neji. -We never saw neji in the infinite tsukiyomi-. The first time...
on page 11 i think it was, on one of the panels that says its time to become one to madara. that person is totally kaguya with the byakugan and the cool sharingan+rinnegan third eye.
Based on hagromo talking about how he made sure that ashura would become the successor of ninshuu. maybe this how the hokage spot will free up for sasuke
Who could use it?
karin- uzumaki , she could potentially know it
(most likely) orochiimaru
naruto could figure it out with the guidance of minato , maybe tsunade