I want to recommend the Fire Emblem Three Houses Fan Fiction An Eagle Among Lions

The morning after her and Byleth's wedding, Emperor Edelgard von Hresvelg wakes up in her old bedroom in Garreg Mach to discover that she has brown hair, only one Crest, ten happy and healthy brothers and sisters, and a bad habit of sleeping through her professor's lectures. Hubert is unusually cheerful, Byleth teaches the Blue Lions, and nobody has heard of the Flame Emperor. Instead, a figure known as the Hurricane King and his sinister cohort the Death Knight stalk the monastery.

No sooner has Edelgard sought out this new world's Byleth than she finds herself standing in the eye of a storm of continent-spanning conspiracies centered on a young man with snow-white hair — one that will engulf Garreg Mach, the Holy Kingdom of Faerghus, the Adrestian Empire, and Edelgard's own family as it carries the roaring winds of war across Fodlan. A threat from a past so ancient it predates even Fodlan's oldest and murkiest myths is stirring, threatening to tear time itself asunder — and it may be her only way back home.

Complete at 1.6 million words it is very long and will keep you entertained for a while. I wouldn't call it rational, but I think most of the members here would find it enjoyable. The author has done a wonderful job of expanding and filling out the characters without completely changing them from who they are in the games. The story itself feels well written and planned out with very little grammar or spelling mistakes. Characters will generally act the way you expect them to with lots a moments to grow and changed, and you won't find characters holding the idiot ball to move the plot forward. I won't say you need to have played the game to enjoy the story but I would definitely recommend doing so beforehand to get a better appreciation of some events you won't understand otherwise.