Gotta share this one. I read a lot of fanfiction, and this one is currently sharing the top spot with the best fics I've ever read (and it shows because this fic was published just last year and it's already the most liked and top rated One Piece fic on the entire site).
Anyway, plot. It's a pseudo-crossover/SI with One Piece and Touhou. Over 40 years before Luffy's adventure starts a girl who ate a devil fruit that gave her the power of Houjuu Nue meets Kaidou, and together they join the most evil pirate crew in history, the Rocks Pirates. Hijinks and crimes against humanity ensue.
The comedy is absolutely top-notch and I've burst out laughing several times. The fic feels almost like something Oda could've written with how good the quality is
Reminds me that I got to start reading like this, but my Japanese is still low level. Need to persevere.