The only one I've encountered so far is that Fallout 3 crossover where Luna is shipped with the protagonist of the game but I want some based directly in Equestria.

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The is one Female Human x Princess Celestia fic called 'The Princess'.

Hm, yeah, I'm reading it right now.

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You could try browsing these groups:
Human Girls in Equestria
FiE
female human x pony
Women in Equestria

More of slice of life stories with no romantic/sexual relationship but I really like Admiral Biscuit's "Sam and Rose Stories". Sam is the female human and Rose refers to Roseluck. Very well written.

https://www.fimfiction.net/bookshelf/615255/sam-and-rose-stories

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What level of content are looking for? Do you want a long or a short story? Something quick and dirty, a slice of life, a slow-burn romance, or are you taking any suggestions?

Does it count if it's a hman woman that gets turned into an alicorn stallion because she got fucked over by a god?