The half mermaid Luffy ideas give me ideas of comedy and miscommunication
What if Luffy was (maybe just when dry) human passing except for scales on his feet and legs?
Thing is, everyone else thinks it’s a Political Statement that he never covers those up. Wearing shoes and pants would be hiding, and he’s not going to do that!
Of course the truth is Luffy just hates socks and loves shorts. Jinbei in particular is baffled to learn this information
So is Arlong, whose kept trying and failing to convince Luffy to join him during their fight, and only got really angry when Luffy (having learned like a handful of basic mermaid bedtime stories as a child) responded to Arlong claiming to be the new Sun Pirates by saying:
“Fisher Tiger broke chains. He’d hate you.”
Other than that, the entire series is literally the same except people fish Luffy out of the water last when he falls in (he can’t drown) and a few bigots dismiss Luffy a bit more than cannon
Luffy also heard stories of one specific food stall on Fishman Island as a kid, so he has an additional goal to visit it when he goes there. This has no impact on the plot, it’s just funny that the thing he knows most about the island is related to food
Yeah! I think, the mermaid!Croc AU doesn't have Luffy who can transform. Maybe if he unlocks gear 5. So human passing merman Luffy would be interesting, especially if he retains some features that make people realize that he is not entirely human.
Of course, as you said, with Luffy displaying his scales is not a political statement at all, he really just doesn't like anything restricting his feet! Crocodile might squeeze his (just going to use he/him pronouns for now until we have a vote about Croc in the mermaid Croc AU) fins into dress shoes and look stylish in it. Doesn't mean Luffy has to do the same!
And if Crocodile grew up as a mermaid on Fishman Island (maybe in the Fishman District because the child of the Sea Witch and a human pirate might be a somewhat suspect individual, but also a novelty, so I bet Otohime would want to chat to Crocodile about the surface), Luffy would have heard some things, including Fisher Tiger. He really could get under Arlong's skin with any comment about Fisher Tiger. What does this kid know? This spoiled brat who turned his back on his blood? Whose mermaid/ merman/ fishman parent turned his back? Who even the sea rejects! What would he know!
Haha, of course Crocodile would tell Luffy about food down there and the kid would latch onto that. Let's just hope that the food stall still exists 20 or so years later *lol*
The relationship between Jinbei and a partial mermaid Luffy would also be a lot of fun, especially if Jinbei knew Crocodile beyond them being fellow (ex) warlords.