So, before I start with the actual idea, here is a little Backstory on the character before our MC will take over.
{Backstory}
Izuna Uchiha (Just like the name but you can change it), the younger brother of Shisui, seen as many as another prodigy. The same age as Sasuke Uchiha and a close friend of his. He awakened his Sharingan on the day his mother died. (Idk when Shisui's parents died, Just needed a reason to give the MC the sharingan, he was also 4 at the time.) After his mother's death his only living family was Shisui who took care of him after his mother died. Before Shisui's suicide, he asked Itachi to spare his little brother. On the day of the Massacre, Itachi told him that he was the one that killed Shisui along with putting him under tsukuyomi and showing him an illusion of Itachi killing Shisui in order to help him awaken the sharingan, however he didn't kniw that he already had the three tomoe sharingan, before he passed out, his Mangekyo Awakaened however due to the stress and all the information he got, the boy dies but at the same time another soul takes over his body.
{Backstory End}
When our MC awakens he find himself in the hospital with no memories of what happened or where he is, after a minute all his memories resurface and he realises that he's in the world of Naruto.
As soon as he gets discharged from the hospital, he goes to Sasuke Uchiha to help him out in his time of need. This will lead to them basically becoming as close as brothers.
Eventually they start living together and start practicing while competing against each other.
The MC is the same age as Sasuke so he will he in the same class.
He graduates the academy with the best results with Sasuke being right behind him.
His gennin team will be: Naruto, Sasuke and himself. (Fuck Sakura) He will be placed in this team because of the same reason why Sasuke got Kakashi for his sensei.
They pass the bell test.
Orochimaru will be after one both of them now.
Our MC already has the Mangekyo Sharingan so in order to get the Eternal Mangekyo, he needs to get back Shisuis eyes from Danzo and Itachi or his second best option would be to recover Fugakus eyes but who knows what Danzo done with them since he didn't know Fugaku had the Mangekyo.
(Give him some cool Mangekyo abilties, Kamui in his right and Tsukuyomi in his left?? Or create some original ability. His susanoo will be pure black.)
During the chunin exams, he warns the hokage about Kabuto.
He encounters Orochimaru in the forest of death however they manage to escape due to his mangekyo.
After the preliminaries, The MC will tell Sasuke the truth about the Uchiha Massacre so he doesn't go thinking about abandoning the village.
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A/N: So this is just the foundation, the rest is up to the one who will use this idea! HAVE FUN!