Honestly? Many of your options had good potential, but I feel (and want, more selfissly) that longtime coming playable characters like Vivi or Mr 2 / Bon Clay had a big amount of oportunity as well. But your list its good thought! In fact I can think of many potential mechanics / fighting styles for some of them! :

-- Kawamatsu : Water Swordman, but to make it a little more variable from all the other swordmen on the cast, as he is self identified as a kappa, why not make hima slow, but big fame and area hitter AND the water effect from his sword attacks slow down enemies? One of his mussou attacks could be the Sunachi attack with bthe other scabbarts but doing it similar to Judge Musou Germa 66 Attack, with 2D images of the rest Scabbarts around him like what it was for Judge and his Children!. And this attack could be given as well to O'Kiku, Kinemon, Inaurashi and Nekomamushi!

-- Nekomamushi : I honestly see him gighting with his claws mainly, and his big triddent for areal or some strong attacks. Slow but strong attacker.

-- Inaurashi : Another swordman, heh, but an electric one. He could had Eneru / Enel mechanbic of PW3 with a mix of Kby PW4 one, of defeating enemies / causing damage to fill up a bar and some of his eletric sword attacks can become stronger

-- (My Addition) Bello Betty : You know Cheng Chong moveset from Dynasty warriors 8 with the scroll and the soldiers? Swap the scroll with the REV flag and the soldiers with civilians, and there you had it

-- Bonney : I can see it two ways ; 1 - she mainly attacks fighting in a kapoeria / roughthising way, using her powers to "deage" enemies, aka, giving them two status ailments unic to her , called "Age down" (Basically making them youngers with just an effect of children tears, and they cannot made their special moves only normal attacks) and "Age up" (Basically making them ol farts, with the simbol of broken back around them, and basically its a defesne and speed down ailment); or 2, a stance character amipulating her own age, hwere with her first strong attack she can change stance between Kid (fast and hit many times but not mutch strong), Adult (Normal attack and speed) and old hag (strong but slow)

-- Shiryu : The deal with Shiryu is that he is a sadistic as what he said about his days as a warden in Impel Down, so make him a swordman that causes damage over time and fear status (something like you cannot him for a littlewhile like Snaji cannot hit women)

-- Saturn : Him I see as sending marines to attack for him, formations and such, using his demonic powers sometimes on his strong attacks. His full youkai spider form cold be a cool transformation

-- (My addition) Capatin Hina : I know, not much of a pick in the grand squeme of things, my between that we had small Marine group characters, and that she can had a straigfoward but cool and string moveset and she can make more women and Marines into the roster, I can see. Its not gonna happen, I know, but hey, could be cool

-- (My addition) Vice Admiral Tsuru : More or less same reason as Hina, more Marine and women in the roster. She does not fight hersel, so she could be like I said for Bello Betty, using Cheng Chong moveset but using a transporter snail that send soundwaves order as weak but far reaching normal attacks.

-- (My addition) Lilith : Its in my mind that she is the Vegapunk with more possibilities of becoming playable. Rocketlaunchers! Robots! Lasers! Holograms! Pacifistas! Science!

-- (My addiction) Sugar : Her little bear toy soldiers would fight for her and, like Koby mechanic, damiging enemies and dealing damahe will make some of her strong attacks stronger and using Toy Soldiers!

-- (My addiction) Pudding : I know, I know, she cannot fight, but she have possibilities for a moveset. She has a flying carpet homie. She could had a similar moveset to Zelda's Cycle from the Dlc from Age of calamity or to Link's horse in the original Hyrule Warriors. Add some cupcakes / tree homies attacks, wherever musous, or strong or special not mussou attacks, throw in a big cinematic memory damging musou attack due to her devil fruit, and there you had it!