My dream One Piece game should not include the Straw Hats for the most parts.
I love the world, the sense of adventure and meeting new places and people. So instead of playing with Luffy, you can create your own captain and visit mainly new island. The game would be still part of the main plot, since you're playing the thirteenth Super Rookie, who just didn't made it to Sabaody Archipelago in time. While each of the original crews took one of the seven routes of the Grand Line, your captain got a broken Log Pose and moves between the different routes. This puts you either against or allying you with the other rookies. Each Island its it own story with a bigger threat following you (like some marine captain, who is after you).
On the way you also meet different people, you can chose to hire for your crew. Each crew members gives you different abilities in battles, solving puzzles, etc. Your captain can have different classes at the start, which would be different Devil Fruits or even no Devil Fruits at all (like a sword fighter). You can level them up over time, get better moves and build your own ship.
This would be the first part of the story. At the end before reaching Sabaody Archipelago, you fight the Straw Hats and lose. You're crew gets splintered around the world and you miss your place as one of the super rookies. Next game would be about fining a new crew and have your revenge against Luffy. At the end of the game series you will become a member of the Straw Hats fleet and be part of the big final of One Piece.