How come Kakashi doesn’t take Obito's eyes since Obito dies toward the end of the war? Also, later in Boruto, Sasuke loses his left eye. Is it possible he can take Madara's Rinnegan since he was fully revived? Sasuke should be able to locate Madara's body assuming the Leaf buried all of the dead shinobi after the war.
Initially I thought of putting a comment but it was getting too long, so I'll put an answer.
In Boruto, Sasuke did lose his Rinnegan and but locating Madara and taking his Rinnegan is not feasible because if you remember towards the end of Ninja War, Madara turned into Kaguya Otsutsuki who was sealed by Naruto and Sasuke. The ten tails was split and so was Madara's body. After taking so much toll on his body, having being turned into Kaguya with his body handling so much power and then getting all ten tails removed, the body was rendered useless and so was his rinnegan (if you notice the last moments of Madara, there was no Rinnegan). It was not shown what happened to Madara's body after his death. Did Hagoromo took his body after all were summoned in the real world?
So, How can Sasuke get a eye (or if possible a Rinnegan)?
Let's look at the ways:
- Amado: Amado is an amazing scientist and can also create Dojutsu. Amado created a special pair of eyes for Delta, which could absorb and release ninjutsu. This Otsutsukis have shown similar abilities, maybe they were the inspiration for the eye. Amado could potentially create a good Dojutsu for Sasuke to use; for which he could take a sample of Kawaki and Boruto’s DNA as both of them have Otsutuski genes mixed with theirs. This could help him to create a better dojutsu or maybe create an artificial Rinnegan.
- Shin Uchiha: Shin Uchiha was a test subject of Orochimaru, who grew to praise the Uchihas, especially Itachi Uchiha. He has multiple pairs of Sharingan, so there’s a possibility that Sasuke could use one of them. A wild guess in both of these possibilities could be that Sasuke uses one of Shin Uchiha’s eyes. Or, a Sharingan from the Laboratory to use the Izanagi, preserving his Sharingan in the process.
- Obito’s laboratory: Obito has a secret laboratory with multiple pairs of Sharingans. Sasuke could use one of the Sharingan preserved in the lab.
- Sakura's healing ability: Sakura can be of help at healing any damages.
There was also one other method: Naruto's Yin-Yang release which he received from Hagoromo Otsutsuki to heal Kakashi’s eye in Naruto Shippuden but since Kurama died, this won't work.
Does Sasuke even need Rinnegan?
Although, getting Rinnegan means attaining godly powers, it is not necessary that Sasuke have to have Rinnegan. Don't forget the fundamental powers of Uchiha, the Sharingan. Sasuke was too powerful before even getting the Rinnegan. Sasuke currently has access to his Susanoo, genjutsu, all forms of Chidori, Fire and Lightning release and other signature jutsu. Now the question is since he has Sharingan, can he still get the Rinnegan powers?
In the words of Tobirama:
the root of Uchiha’s powers is their love, greater than anyone else’s. But when they lose someone they love, their love is changed into hate. And this emotion of theirs is reflected by their eyes. That’s the reason it is called "the eye that reflects the heart."
This means that the powers and all the Uchiha abilities come from within a Shinobi’s blood (Kekkei Genkai). The eyes are nothing but the projectors for that power. In this case, Sasuke has all the abilities of Rinnegan within him since he’s the direct descendant of Indra and thus Hagoromo; he just needs a tool to access it which will be Rinnegan eye.
But what I've heard in Boruto, Sasuke hasn't been on the spotlight as he is having a hard time with the bosses. Sasuke has been using space-time ninjutsu quite a lot lately which is very chakra-inducing and hence, he is getting tired and couldn't keep up with the enemy. This is putting him in a disadvantage. If he can get to his old fighting style and using his basic jutsu, without depending on godly powers, he might get good in the future.
Also, since Obito died, his Sharingan that was implanted to Kakashi got deactivated and hence his eyes returned normal.
Obito died by disintegration, so there’s no chance for Kakashi to obtain his Sharingan. In fact, it was quite a big deal for Obito’s soul to briefly inhabit Kakashi’s body and give him both his Sharingan in order to assist in defeating Kaguya.
I think it’s also safe to assume that Madara’s Rinnegan are no longer viable considering the amount of time he’s been dead (even if it is true that Madara’s body exists somewhere and is accessible by Sasuke).
But that aside, neither Kakashi nor Sasuke have shown any inclination since the War to actively seek out powerful jutsu, whether ocular or otherwise.