Can't say whether or not I'm interested yet, as I don't know for sure if I'll have time for another RP, but I feel like I could offer some advice.

What seems to get most fans of One Piece to stick around are three primary things--the fights are probably the biggest one, followed by the sense of exploration, and then the characters. To make interesting fights, you have to have interesting villains. And when I say interesting, I don't just mean their powersets and fighting style, although that's also a big part of it. A good villain has to have impact and importance. Impact means that their actions affect the world around them, and importance is relative to the good guys. A mad scientist you've never met, who wants to take over the world, may not be as "important" as the petty mobster who forced a character's father/best friend/lover to go into debt and now threatens them when they don't have the money. You have to have a villain who can put up a fight, but they also have to be a villain that the characters REALLY want to fight in the first place. If they are recurrent villains, however, you have to consistently make them threatening so they don't end up like Team Rocket.

To establish a sense of exploration is hard in an RP format, I've found. One Piece has a lot of islands you can build on, and you can always put a lot of work into describing an environment or use nice pictures to get some visual aid going, but it's hard to synthesize player goals with "a place" as opposed to a "person or thing." In other words, the players are most likely going to be on an island to fight somebody, get a special item, or do something mundane like stock up on supplies. If you can come up with ways to make those goals tie into the environment you want them to explore, it'll probably be easier--for instance, to fight someone, maybe they have to find the hideout that's hidden in the vast jungle. Same thing with the item, maybe it's sealed inside an ancient temple. Even the gathering of supplies--maybe one of the characters is like Luffy, a huge glutton, and they hear about this island that has these specially raised cattle that make the most succulent steaks on the high seas. Then they find out that these steaks are insanely expensive, and the only way they might be able to get one is if they help the local cattle farmer drive off some rustler bandits. 

Basically, turn "exploring the environment" into an adventure in itself.

Finally, the characters. This will heavily depend on your players themselves and their skills as writers. But, one thing you can do to help is communicate with them regularly, and often. Especially in this case--Character Arcs. One Piece has an Arc for each and every one of its primary crew characters that also serve to establish bits and pieces of the overall story. If you encourage and talk to your players about their backstories, their character's personal beliefs and likes and dislikes, you might be able to help them come up with ideas for their own arcs, which you would then execute alongside them, or you could create an arc based around them by yourself and just ask them if they're cool with you creating events/villains/NPCs that are related to them in some way. The only problem with this is that, in an RP, everyone wants to share the spotlight equally. So you'd either have to come up with a set order that everyone agrees with--"Okay, we'll do Bob's arc first, then Jane's, then Jim's, then Sally's"-- or else you'll end up with multiple arcs running concurrently, which is very hard to handle both from a GM and player perspective.

As for your question about an NPC captain or a PC one, keep in mind two things about Luffy as the captain of his own crew--on the one hand, he doesn't really do "captainy" things most of the time. On the other hand, he's ALWAYS the one who solves the primary problem. Like any main character in any story, it falls to him to handle the biggest threat while everyone else has their own opponents--which is another thing I should have discussed in the villain section. When you make a villain, make sure to give them plenty of lackeys so everyone can have their own 1-on-1 fair fight instead of a gangbang chaotic mess of everyone attacking one villain at once. But anyway, whether or not the captain is an NPC or a PC should address those things. Remember, in an RP everyone shares the "main character" slot. So your captain can't be the one to resolve all the problems. At the same time, the captain is the one that runs the ship. You'll need to decide "to what degree or standard does he hold order on his/her ship?" And depending on those factors you might find it easier to decide if it should be an NPC, a PC, or even a character made by another player instead of the GM.

Hope this helps and wasn't too long winded.