Im going to try my hardest to not make this sound like bad fanfiction.
the episode count would be around 10-15 episodes. and take place inbetween PH and Dressrosa
On the waters near the DF negating island, the crew awakens only to find Luffy, Robin, Chopper, Brook, and Law, and CC feeling sluggish. Suddenly without much warning a Strong new World Hurricane comes out of nowhere Violently shaking the Sunny Toward the island and Blowing Luffy's hat away in the process. Luffy fails in grabbing it, because to his shock His arm wont stretch, and thus the crew learns of their predicament. Luffy loves his hat too much to simply abandon it, and so begins a hunt for it on an island that negates DF powers. On this island there is are large feral Animals who can kill you if you don't know Haki. Deep within the woods of the island a base is found where a small group of Local milita men (this is the New World so yes they are extremely proficient in Soru and Haki techniques) and scientists are discussing a way to completely demolish Devil Fruits from the rest of the world, and using the special Tree pollen found on the island they wish to make a powerful Pathogen bomb and unleash it on various islands of the grand line as well as sell it off. As the strawhats catch wind of this they must now stop the Army and their scheme before the balance of the world is Shattered. and just what has happened to Luffy's hat?
this leads to another small storyline where luffy gets his hat back but due to his skin absorbing the pollen stuck in it he gets extremely sick and begins to dream about what were to happen in he fought the villains of the past with his current DF Powers, during his treatment. a flashback arc on how far he's come
this entire thing is about 25 episodes and would be a nice means of spacing the two arcs with nice-flowing Action.
Just for Lolz i'll call it Muggle Island.