Fics like Concourse of Realities (AKA "I throw all my blorbos together and hope they play nice")

Through means yet unknown to her, Monkey D. Luffy is transported to a strange, grassy land, and meets some stranger, but substantially less grassy, people. And a slug, you should not forget about the slug.

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The Will of D: Divided Paths by Thewilloftomorrow (One Piece (Anime & Manga), Naruto (Anime & Manga))
The Straw Hats’ Watchdog by Navisland (Katekyou Hitman Reborn!, One Piece (Anime & Manga))
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A Damned Blond in the Ocean by AmusedLight (Naruto, One Piece (Anime & Manga))
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Villain Rehab Luffy | Luffy gets lost in the world of mha by Bizzy_Glitch (One Piece (Anime & Manga), 僕のヒーローアカデミア | Boku no Hero Academia | My Hero Academia (Anime & Manga))
Sinker, Line, and... by NailPolisher (One Piece (Anime & Manga))
Set Sail! Into the Rising Dawn by KayMorse (One Piece (Anime & Manga))
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Stormborn by tongue_blisters (One Piece (Anime & Manga))
Sea of Souls by Black_Victor_Cachat (One Piece (Anime & Manga), Bleach (Anime & Manga))