Ordered more or less chronologically. There are a few educated guesses here, but most of these just cater to my personal likes, feelings and/or interpretations, sometimes inspired by fanarts and fanfics. However, canon is mostly respected unless proven otherwise.
Also, take a look at the Senju and Uchiha clans that I made, please. It may help you not to get lost with some things.
*(content will be cut in 4 parts because too long)
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1. Since Hagoromo had two sons, Hamura instead fathered two daughters. One remained on the Moon (Otsutsuki clan ancestress) and the other returned to Earth (Hyūga clan ancestress).
You can check out their names in my fan-named characters, along with many others.
2. The Hagoromo clan originated from members that sided with Indra and followed him into exile. That's how they were still allies with the Uchiha thousand years later.
3. The Kaguya clan were somehow spawned by black Zetsu (from white Zetsus) thanks to Kaguya's genetics from itself.
4. All five of Uchiha siblings were named after the Northern Shinshu range, in this birth order: Madara, Kurohime (♀), Togakushi, Myōkō (♀) and Izuna. They were only one year apart each (meaning that when Izuna died, both Hashirama and Madara were 28yo and Tobirama 27).
5. Hashirama and Mito had a political marriage. They didn't marry for love, but over time they managed to develop an actual caring and affectionate relationship. They had two children at least (pre- and post-bijuu.
6. I relocated Uzushiogakure because it bothered me that Kishimoto put it on the same island as Land of Waves. I placed it here instead.
7. Hashirama and Madara were definitely in love. Perhaps it was only platonic, but I take it as romantic.
8. Madara married Naori out of admiration for her heart and will to sacrifice for her last brother, Naka, which reminded him of Izuna. They had a son and daughter.
9. Setsuna and Kagami were sons of Madara and Izuna respectively.
10. Izuna died at 24yo, which is more than old enough to sire a kid. He got some Fuuma woman pregnant on his very first experience even before his hatachi, out of an act of rebellion. She died at childbirth, so Izuna was able to get baby Kagami with him.
11. The little girl who tripped at the academy and Madara helped was Biwako. She was originally a Senju before marrying Hiruzen (because her clothes are just so similar).
12. If Madara had been loved enough, Zetsu wouldn't have been able to brainwash and corrupt him (if Izuna hadn't died… if his clan hadn't shunned him… or if Hashirama had actually done something to stop Tobirama's slandering).
13. Most Uchihas can't get drunk. Their potent katon nature evaporates all alcohol from their system before it can take effect. At most, they only get a bit tipsy for a very short while.
14. Uchihas don't bury their dead, they cremate. Coffin pyres as poetic imagery of returning to the fire, or becoming one with the fire... something like that.
15. Hikaku Uchiha was made clan leader after Madara left.
16. He was pretty much coerced (by Tobirama) to marry Touka Senju for politics, and he did it dutifully in order to help stabilize the clan's position in Konoha. They only had one son for the sake of appearances. Sadly, this caused that neither of his parents were too thrilled by his existence, though they treated him well.
17. Hashirama was invaded by his mokuton in a sort-of autoimmune disease that made his body turn into the tree in the school yard. Yes, the tree where Naruto's swing was hung on. That tree which was the only thing to survive Pain's attack? If that's not mythical strength I don't know what would be.
18. Tobirama is albino (he's not half-Hatake nor anything like that).
19. He had UST with Izuna and didn't know how to deal with it, hence Uchihan frustration issues forever (yes, I admit that I've been corrupted by all those fanarts and fanfics... but I don't regret it).
20. Tobirama only took in Kagami (Izuna's son) to get his hands on an Uchiha insider for spying.
21. He killed Orochimaru's parents (as implied in Jiraiya's novel).
22. Senju clan intermingled with other clans in Konoha in order to promote unity and stability. That's how they "disappeared", meaning most of the current clans probably have a Senju ancestor.
23. First mizukage Byakuren was an outstanding smithy. He forged the Seven Swords of the Mist in an attempt to compete with Kumogakure's Hagoromo treasures. This feature was what granted him being chosen as the First.
The swords were gifted to the seven most prominent clans in Land of Water in order to convince them join in the founding of Kirigakure.
24. Third mizukage Saizou Sanada was born blind and he developed as a sensor since pretty much birth. His sensoring ability remains unparalleled until today!
The pearls he wore on his head actually served to block tiny and unimportant amounts of chakra (like those coming from plants and animals), in order to help him focus only on actual possible threats.
25. All minor lands and villages accepted! The map would be too empty otherwise.
*(continues in part 2 - part 3 - part 4)
