This should probably go somewhere else... but anyway.. here are some of mine with my own interpretation of how they would look, feel free to add new and fresh ideas and even spin what's written by others.
"Box Jellyfish" Fishmen. - He'd be Admiral/Yonkou level. Look pretty normal but his entire body frame would be made of tentacles strung together in the shape of a man. Translucent skin tone, and would be deadly as he'd have almost no vital points, and be deadly as a logia to the touch. Probably would use Fishmen Karate as well, just because.
"Pistol Shrimp" Fishmen. - I can't express how awesome this would be to see.
"Electric Eel" Fishmen.
"Vampire Piranha" Fishmen.
I really like the box jellyfish fish man idea. Could be a force to reckon with in the op world, but I'm not sure how well its traits would actually translate into the series. We've seen the blue ringed octopus fish man and this creatures poison in rl is extremely deadly and is said to have no known cure, but luffy shrugged it off like it was nothing.
Haha, cant stop laugh about the thoughts Luffy have about fishmen
I wouldn't mind seeing a flying fish fishman, though perhaps a seahorse fishman would be interesting, or a sea turtle or lobster or something, there is a lot of potential really.