After taking on the 20 S rank ninjas and taking out 17 of them to save his squad and his successor, Tobirama lived his last moments, atleast thats what he thought.
When he regained his consciousness, he found himself stuck in a body of a 7 year old Uchiha just after the Uchiha Massacre. (Of course you could have him be born as a child and be Naruto's age but i feel like that would change the story too much.)
Not knowing why he was still alive, he decided to live on but after seeing the state of the village and how everyone treats their Jincchuriki, he decides to take matters into his own hands.
You could also have it that the body that Tobirama took over already had the 3 tomoe Sharingan and that the body had awakened the mangekyo after witnessing Itachi kill his family. (This is honestly the only way i see Tobirama unlock his mangekyo, unless you have him unlock it after witnessing the death of Sarutobi)
(Don't make him tell others that he was reborn cause i feel like stuff like that ruin some stories, he should only reveal his true identity to Tsunade)
Tobirama also noticed that Sasuke and Naruto have similar chakra to Madara and his elder brother Hashirama. (Indra and Asura chakra)
He decides to train both of them to help out the lonely jincchuriki and to make sure that Sasuke Uchiha doesn't become the next Madara.
Team 7 - Kakashi Hatake, Naruto Uzumaki, Sasuke Uchiha, Tobirama Uchiha.
(Obviously you would want the Uchiha's to be taught by the only other sharingan user and Kakashi himself could have requested for Naruto)
They pass the bell test on their first try after Tobirama convinced them to work together.
Seeing the potential in his genins, Kakashi decides to take their training more seriously then he would of had in canon
Tobirama Uchiha sounds kinda cold ngl 🥶