After defeating kaguya sasuke wil take her rinnegan and transplant in his left eye.thats why sasuke awakend the juubi rinnegan eye and only one eye.he surely won't stand with only a rinnegan and somehow he willl demadge his MS during the battle.After Kaguya will be defeated we will have 9 biuus and a rinnegan waiting to find a host.Naruto for the bijuus and Sasuke for the Rinnegan than we will have the final battle with both characters powered at max.
Re: Naruto and Sasuke next power up and final battle.
I don't think that'd make much sense. The reason he only has one Sharinnegan is for the same reason Kaguya and Hagoromo do. He just doesn't have enough Bijuu chakra to have unlocked it in his forehead, since a jinchuuriki is one with their tailed beasts, not just using their chakra. Sasuke right now has bijuu chakra within him thanks to Hagoromo, but he's not a jinchuuriki unlike how Madara and Kaguya are.
He'll stick to one fSharinnegan eye, but I do think he'll gain more abilities, probably turning his right eye into one of the basic, white Rinnegan (though unlikely). I'm of the opinion the power that Sasuke and Naruto gain to beat Kaguya will come from Hamura if they do indeed get a power up (they technically don't need one. They just need to connect with their sun and moon seals).