So I just thought to myself when I was playing Unlimited World Red "Wouldn't it be super awesume if there was an open world One Piece game?" And I began to get some idea's.
Like the ability of Character customization
Being able to modify your ship
Find Devil Fruits and decide if your gonna eat one of them or not (Super rare ofcourse)
Go to famous Island's and explore.
When you become a high enough Bounty Level you can claim territories.
Maybe an Online modus like GTA V has.
Skills that you can learn ofcourse (A bit like Skyrims way of combat but then with skills)
But there are many more things that could be added and maybe it will come one day!