I debated for a long time about putting this up. It took me a very long time to piece together her major plot points (mostly due to my own emotional issues, going through stages of grief) and most of it is so disjointed that it isn't really worth putting up. But, there are some amazing plot points that are fleshed out and worth knowing, especially to tie up loose ends. This will read more like a discussion/summary by the way instead of a proper outline or chapter. Also, not edited so apologies for any typo stuff.
WARNING: Some of the following content is very dark with issues such as murder and rape mentioned. Read at your own risk. (No, Fay does not get raped)
1. Scotland and where Fay must never go
So remember when the toads asked Fay whether she was from Scotland? Oh man is this a can of worms. So, approximately 300 years ago, before the founding of Konoha, before even the founding of the Uzumaki clan, a young woman from Scotland and her 2-year-old baby boy are transported from Scotland to what is present day Land of Earth. No name for the woman or boy named, probably my cousin hadn't decided on one yet. However, one little tidbit mentioned was that they'd been transported during what is known as the Little Ice Age and were thus very famished and weak.
So bad luck has it that they were transported right near the camp of a bunch of shinobi. Shinobi who were the precursors to the Senju clan. Remember at that time Hashirama isn't even near born yet. The clan, actually all shinobi really, are in a population crisis. Not enough women, not enough children, too many dying in battle with other clans.
Ugh, how to put this. Well, my cousin probably based this on real historical events, but to bolster their female shortage, shinobi clans were going around kidnapping 'wives' left and right from civilian populations. When Scottish lady and son appeared, the shinobi were probably thinking, along the lines of, oh what luck that while searching for females that one would just seem to fall out of the sky.
Anyways, seeing the famished state of the little boy they coldly decided he wasn't useful. Despite the lady's tearful begging (not that they understood her language) they did the 'merciful' thing and killed the boy to silence his pained screams from the blood boiling effect while giving her a chakra infusion to save her. Naturally, this completely messed up the poor, poor lady's head.
Years pass, and her blood's amazing effect on seals was discovered. They also find out that all her children (she has 3 now) not only inherited her red hair but have amazing gift with sealing, potent life force and huge chakra levels, just like their mother.
Well, I won't elaborate on the cringe worthy aftermath of the shinobi discovering the lady has what they think is a valuable kekkei genkai. Essentially, she had a lot more children, none of whom she has any contact with. Her children don't even know she's their mother really, which is so tragic.
Naturally, the lady has gone completely dark and crazed from the terrible abuse. She spends the rest of her life plotting vengeance. When she's an elderly woman, she finally completes her tool of vengeance, a seal that was to destroy the world. Using an obscene amount of her own blood she summons the Shinigami. Yup, you read this correctly. The original creator of the Dead Demon Consuming Seal is the Scottish lady from our world. This God of Death though is not like the one Minato summons. Nope, this motherfucker is HUGE, like bigger than the 10 tails.
So this Shinigami starts killing everyone, and the Senju precursor shinobi realize that if left to do its task the Shinigami will literally kill every last one of them. In the end, six of the lady's own children and grandchildren sacrifice their life to cancel the seal and essentially seal away the Shinigami before it devours mankind. To do this, they also had to kill their own mother (who they didn't know was their real birth mother) as she was controlling the Shinigami. However, the huge Shinigami seal remains, locked, but not destroyed; at least not until it consumes enough life force to 'fill its belly'. The only way to unlock the Shinigami now is for a lot of really potent blood to be spilled onto it. In other words, a ton of blood from the lady's children, or from another being from our world.
The Senju precursor clan splits after this event. The red haired legacy of the lady and their family remain behind to monitor the seal and make sure it is never unlocked. Where they settled became Uzushiogakure and they became the Uzumaki clan. The other half of the clan move west into Fire Country and become the Senju clan.
So, the toads were telling Jiraiya to make sure Fay never, ever goes to the ruins of Uzushiogakure because she may accidentally unlock the Shinigami with her really potent blood.
Whew. That was really long and hard to write. Moving on.
2. Who does Fay end up with?
Fay ends up with Death Cat (Yamato). I personally was kind of rooting for a Fay and Samehada pairing, but well it was not meant to be I suppose (I kid I kid). Essentially, Death Cat already has a minor crush on Fay by the time he had to fake her death, but he was being all professional about everything - hence the guilt and gift giving. Later as he gets to know her as Yamato, he starts to get even more attached.
Well, Danzo has the brilliant idea to get Fay hitched to a Konoha shinobi, and thus securing her sealing abilities - hopefully through her children as well. He wants her to marry one of his loyal men, so assigns Fuu Yamanaka to woe her. Well, that goes about as well as you would suspect with Fay being completely oblivious to Fuu's emotionally constipated advances, while everyone around her becomes aggressively overprotective. Death Cat especially gets all jealous and affronted by the appalling flirting, thus pushing him to act on his feelings.
Toward the end of the story, Fay and Death Cat only have begun a fragile, tentative relationship (confession, hand holding, cuddling). However, Death Cat still hasn't told her that he's also Yamato (Fay knows already of course, but he doesn't know that). He thus feels conflicted and guilty as hell for lying to her, while Fay waits impatiently for him to just man up and tell the truth. Before he can though, shit goes down in the form of the entire hullabaloo with Akatsuki and Kabuto.
3. How does Kaguya get dealt with?
Yamato is kidnapped as well as Anko by Kabuto and Fay becomes desperate to save them. She somehow learns about the locked giant Shinigami seal, and decides to sacrifice her own life to use it to defeat the enemy. However, no one will allow her to do that and she is physically prevented from leaving. Fay is actually at the battlefield (at a secured camp, however) due to her sealing abilities being so vital to war efforts, plus she absolutely refused to be left at the village, not with Yamato and Anko missing.
Well, turns out that Fay isn't immune to the Infinite Tsukuyomi, but she sure is to the God Tree. When it tries to cocoon her, it implodes and breaks the Genjutsu instead, awakening Fay. Kaguya is full on rampaging by then. Kaguya transports Team 7 and Obito to another dimension, not noticing Fay is awake as well. Well, Fay is just fed up. She has no idea who or what Kaguya is, but Fay is determined to destroy her if that will get things to finally calm the heck down.
Fay, ignoring the Edo Tensei Hokage and uses a seal she has been preparing for a while to teleport to Uzushiogakure. There she awakens the Shinigami with her blood, and orders it to eat Kaguya. Well, my cousin concluded that as the Shinigami is literally a god in this huge giant form, it would have no problem travelling across dimensions. Fay, encased in the middle of the giant Shinigami, kind of like Susanoo, teleports to Kaguya's dimension and proceeds to quite anticlimactically grab and swallow Kaguya. The tailed beasts are released. Black Zetsu freaks and attacks Fay, which result in Fay making the Shinigami consume him as well.
Then. Well. Then Fay dies as the Shinigami consumes her soul as well. Fellow fans, Fay DIES! Oh my kami! Not only that she's stuck in the Shinigami's belly with fricken Kaguya and Black Zetsu!
Like the manga, all get summoned back by the Edo Tensei crew, Obito is still alive though. Team 7 is inconsolable, especially as they know Fay is stuck in the belly of the Shinigami.
Everyone is released from the Genjutsu and Sasuke suddenly has an idea. Orochimaru (yes, who has revived himself a while back apparently) released the Hokage from the belly of the Shinigami, maybe he would do it again and release Fay (especially with incentives). However, there is the fear that this would release Kaguya and Zetsu. After consulting with Orochimaru however, they realize that he would be able to completely control what happens to the souls that are released. Even being able to put them back into the belly of the Shinigami. Essentially, without Edo Tensei the soul of Kaguya and Zetsu would be unable to do anything.
Orochimaru goes ahead with the Dead Demon Consuming Seal: Release, and splits his belly open, releasing the soul of Fay and Kaguya, and Zetsu (who isn't alive to have a soul, more chakra like bijuu really). Naruto and Sasuke seal Zetsu and Kaguya's soul like they did in manga, to make triple sure they can't make mischief.
All are here under the impression that they are just releasing Fay's soul to move on, but Obito has other ideas. He took the Rinnegan eye from Kaguya's body while everyone is distracted/distressed with Fay's death. He's already slowly dying from being forced to use Outer Path - Samsara of Heavenly Life Technique to resurrect Madara. He doesn't have the strength to call a soul or souls back from the afterlife, but does however have the strength to call back to life a soul already in their world, Fays.
Fay is resurrected by Obito, who then dies from the stress of the technique.
Not much information here other than that Fay and Yamato eventually marry and have twin boys who they name Han and Obito.
Thank you everyone for reading. Hopefully this brings real closure to the story that my cousin created.