Quite a lot of people are wondering what if Itachi would have received Rinnegan, let's talk today about how Itachi would have changed after receiving Rinnegan in one way or another.
As you can see from the manga and official encyclopedias, you can get Rinnegan in several ways, and note that I said just getting it, and not specifically awakening, that is, here we will also consider options for the usual eye transplant with Rinnegan. So, the first method is a transplant, according to Uchiha Obito, controlling even one Rinnegan was a problematic task for him, and what can we say about a pair of such eyes. It should be noted that initially Obito wanted to transplant both of Madara's eyes, but it turned out as it turned out. There is no need to talk about Nagato, he was a descendant of the Uzumaki clan and in this regard, a carrier of powerful vital energy and large amounts of chakra, which is very suitable for using someone else's Rinnegan. If you compare Itachi with Nagato and Obito, it will immediately become clear that Itachi would not pull a couple of Rinnegans, since he had a small amount of chakra, only two and a half points according to the official encyclopedia(this is very little by the way). By the way, this unfortunate chakra became the limitation of such a genius. So in this scenario, Itachi would lose one Mangekyo, as he would have to replace it with a Rinnegan. Well, in theory, of course, Itachi could try to activate the Mangekyo on top of the Rinnegan, as Sasuke does, then his eyes would look something like Sasuke's with only three tamoe.
Such ninjutsu as Shinra Tensei and Chibaku Tensei can be removed from the arsenal of ninjutsu such as Itachi, and the use of Gedo Maso would completely kill our genius. You can just remember what happened to Nagato.Now let's talk about the second method - sealing the Rinnegan chakra. This is the type of chakra necessary for the work of the Rinnegan. A similar method was used by Ootsutsuki Hogoromo, while transferring the power of his Rinnegan to Uchiha Sasuke. In principle, this option is slightly different from the first, Itachi will still experience problems when using Rinnegan, but unlike the first option, this method will allow Itachi to use Rinnegan and Sharingan in one eye, as it does for example Sasuke, which is a definite plus, as the practice of Sasuke Rinnegan chakra improves genjutsu. So for example Sasuke easily subdued 9 biju in an instant. It is likely that Tsukuyomi Itachi would have been improved if Rinnegan had been in the left eye, and given the fact that he was a master of genjutsu, this method strengthened him quite powerfully. I can't even imagine what level of genjutsu he would have.
Well, the third way is to wake up. This is the last method, and as a rule the most effective, since in addition to the duration of the awakening, it does not carry any disadvantages. To awaken the Rinnegan, the user must have the power of Uchiha and Senju, or rather the power of reincarnation of the sons of Ootsutsuki Hagaroma, so for example did Uchiha Madara. We can say that Itachi was unlucky from birth, than his brother Sasuke, who is the reincarnation of Ootsutsuki Indra, but this is not so. In the presence of the eternal Mangekyo Sharingan, Itachi will need only one component - the Naruto chakra. He needs to combine the dna of his brother Uchiha Sasuke and the dna of Uzumaki Naruto, and then and only then will he stay Rinnegan, and he could get the eternal Mangekyo only by transplanting Sasuke's eyes, and this automatically gives Itachi Indra's dna. It remains to take the dna of Naruto and voila, but different organisms react differently to foreign cells, so for example, Madara took plus or minus 30 years for the cells of Hashirama to assimilate his cells. How this process will proceed in the case of Itachi and whether he will even live to see the days when he can awaken Rinnegan is unknown. I will also note that Naruto, as well as Nagato, is a representative of the Uzumaki clan, who have powerful physical energy, and accordingly Naruto is also no exception. With his dna, Itachi will significantly increase his chakra reserves, which will allow him to use Rinnegan more effectively, compared to the first and second methods, where Itachi was limited to using Rinnegan only two or three times. Well, finally, I want to say that if Itachi's eye is native, that is, he himself awakened it, then with this method he will spend less chakra on it than if he transplanted it, that is, this method will strengthen Itachi the most.