A/N: This one isn't really original, however, the ones that follow this concept arent well written and have a lot of mistakes.
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Relationship - Karin?
When writing this fanfic please focus on the Uzumaki clan and their fuinjutsus and kekkai genkais. You can give the MC a really pure bloodline so he will have all the strongest abilities of the Uzumaki Clan.
(One thing I always wondered was why Hashirama was the only one with Mokuton, I think that maybe in order to have the Mokuton you need a really pure bloodline to be as closest as possible to Ashura Otsutsuki. Uzumakis are hus descendents so maybe if a Uzumaki has a really pure bloodline they will also get Mokuton? Just a theory tha is probably wrong.)
The MCs goal should be to protect Naruto and to rebuild the Uzumaki clan either in Konoha or rebuuld Uzu.
Reborn as Naruto's twin brother, Menma Uzumaki. he will not get the nine tails.
So, unlike the other ideas i posted, the MC dies however he doesnt meet god. The moment he opens his eyes again he sees destruction from the nine tails attack.
Write about the treatment they get from the villagers and how they survived until they got to the academy.
Write about the MC awakening his Kekkai Genkai (Adamantine Chains), Ive read they also have a sensor ability that allows them to easily track other Uzumakis, give the MC a powerful healing ability.
Write about his academy days, he will obviously be a genius so maybe make him graduate early and get Hayate gekko to become sensei to teach him kenjutsu.
Make him train kenjutsu too (I just wanna see what the Uzumaki could have been if they haven't been wiped out.)
Maybe learn the flying thundergod (Red Flash?)
In the Chunin Exams, you could have the MC rescue Karin and get her transferred to Konoha. Afterall she is a pure blooded Uzumaki.
In the 3 year time skip you could have the MC go search for other Uzumakis that are scattered around the elemental nations and try to convince them to relocate to Konoha where they clan will be based (Unless you decide to rebuild Uzu)
You could have the MC go to Uzu to visit the ruins where he finds a secret room that was sealed away that only an Uzumaki can open where the MC will find the Uzumaki jutsus.
Then you have the Gaara rescue arc and Pain arc, idk about what to do about Nagato so have fun deciding.
Help Naruto control the Nine Tails.
Then you have the War Arc.
And from here on you either finish the fanfic or continue into the Boruto part.