Okay so here is just an idea of a movie. (Everything is pure Fan based and Speculation and is not canon.)
One Piece Film: Will of D.
300 years ago, in the city Chou'Kilith, a massive occult worshipped a ancient evil God from another dimension named Yushinnaga and his horrid minions who once tried to bring about 5,000 years of darkness and drive mankind to the bring of insanity and extinction 10,000 years ago but was banished by a Sea Goddess named Elmia and a mortal named Gremlin D. Griffin, and now intended to return with the add of his human followers, but was thwarted by a group of D. people that barely stopped the ceremony but need a willing person to offer their blood to completely close the portal to Yushinnaga's world. The volunteer was a Monkey D.. The city and all it's secrets sank to the bottom of the ocean, or so we thought.
A group of decsendents from the occult was formed and intended to finish what their ancestors couldn't. They attacked Amazon Lily Believing that there is a trace of Monkey D. Luffy's blood. They can use that to track him, after their Attack, Boa Hancock worried about Luffy took her crew and followed them.
The Occult Group Members:
- Nefil: Leader of the group and ate the Null Null Fruit (Paramecia) which has the power to negate things (Ex. Negate other powers, negate fire leaving only smoke, negate stone leaving only dust, negate life leaving only death), the only Devil Fruit that can block Haki powers. Possesses all Haki.
- Scito: Acolyte who ate the Sting Sting Fruit (Zoan) which can turn him into a moscito. Create stingers, fly at mock speed, track people by sucking blood. Possesses Haki/Kenbunshoku Haki and Haki/Busoshoku Haki.
- Yoga: Acolyte who is a master martial arts user who can create energy based Attacks. Possesses Haki/Busoshoku Haki and Haki/Kenbunshoku Haki.
- Bain: Acolyte who is has enhanced strength, speed and endurance and wields two hammers. Possesses Haki/Busoshoku Haki and Haki/Kenbunshoku Haki.
Meanwhile with the Marines, they discovered the risen sunken city of Chou'Kilith, they tried to investigate but had no success with people going insane when getting close to it. So they decided to wait and see.
Meanwhile with the Straw Hat Pirates, they are enjoying their adventures unit the Occult group attacked them and warned them to go hand over Luffy or else.
Hancock and her crew arrived shortly after, they stopped at an island and met a person named Gremlin D. Gilferd, Luffy and Hancock had a moment together until Nefil Attacked, he realized that he needed Luffy himself to break the seal on Yushinnaga's portal, so he captured Hancock and told him to come to Chou'Kilith.
Gilferd revealed that he is the son of Emlia and Griffin which made him a demigod. He couldn't leave the island when Yushinnaga is not on earth.
The Straw Hat Crew decided to go after them while the Marines learned of what transpired and gathered all Marine Personale, Captains, Admirals, Vice-Admiral, Warlords to the city as well.
After a fierce battle, Nefil tricked Luffy into attacking the portal and thus releasing Yushinnaga and his minions from their world. All out war was initiated, eventually Gilferd arrived and battled Yushinnaga. Luffy continued to fight Nefil in order to save Hacock.
After Nefil and his occult is defeated, the Straw Hats and Hacock watched the conclusion of Gilferd and Yushinnaga's battle, Yushinnaga got the upper hand but then Gilferd sacrificed himself to drag the evil God back to his world and seal the portal. The city once again sank into them ocean. Luffy and Hacock had one final moment before they parted ways.
Thoughts? Your own movie ideas?