The MC gets reincarnated as an Uchiha. He has no cheats and has normal Chunin strength with a two tomoe Sharingan. But after witnessing something mentally disturbing he gets the MS.
The thing about this FF is mainly the powers of his MS.
Left Eye: Perception on the level of six eyes, possibly even better. Results in practically ultimate chakra control and therefore also almost infinite chakra capacity. The main feature of this ability, though shows up if paired with the Right Eye's ability
Right eye: The ability to manipulate/control his own cells. The MC can change his body on the level of fundamental DNA and how his body is build.
If we combine these two, he can practically observe others with Kekkai Genkai and such. This will allow him to copy the way the cells of these are build and therefore integrate this build into his own cells. Therefore he can practically copy every Kekkai Genkai, every body features (Hashirama Cells) and even other MS/Rinnegan abilities. He can also continue to enhance his MS abilities to make them even stronger. Even getting the EMS isn't out of reach by seeing it on others, but Hasirama Cells already takes care of that.
At some point he is the ultimate adaption since he can choose how to adapt, he can copy any ability, he can replicate any body feature, and will remain fresh minded and without fatigue infinitely.