I like the Arlong idea. I even thought up an idea of how it'll play out. (Entirely what-if)
The movie opens with the Sunny sailing through the Grand Line. Franky is tinkering with the Shark Submarine. Robin is reading in the parlor while Sanji is fixing drinks. Nami is outside on a chair sunbathing. Zoro is sleeping. Luffy and Usopp are playing with the cannon, while Chopper and Brook watch. Luffy doesn't know how to work the cannon, and Usopp shows him how by shooting a nearby rock. Luffy, Chopper, and Brook are surprised, but soon they are attacked. Zoro recognizes the attacker as his friend and fellow bounty hunter, Johnny. His partner, Yosaku, is about to die. Chopper deduces that it is scurvy and Sanji prepares a bunch of oranges. Yosaku is miraculously cured (to Nami's annoyance and Chopper's annoyance. Usopp apologizes and Johnny and Yosaku bid farewell to the crew, but drop one of the wanted posters they carried. As they leave, Nami notices the poster and gasps when she sees the picture.
Meanwhile, the crew arrives at an island. Nami agrees to watch the ship (which confuses Zoro, who thinks she is up to something). Nami is left alone on the Sunny. She pulls from a bag in her room an Eternal Pose. It reads: Commi. Meanwhile, Robin, Chopper, and Sanji are shopping. They come across a bounty poster, reading the name "Saw-Tooth Arlong", with a bounty of 200 Million (creative changes, of course). Robin recognizes the name, She remembers that Arlong led a group of pirates, who were formerly under the win of the Shichibukai, Jimbei. Meanwhile, Luffy and Zoro are chowing at the bar, when they are attacked, leading to the obligatory bar fight.
However, the fight is broken up when Usopp runs into the bar, distressed. Luffy asks what's up, leading Usopp to tell him that the Sunny has left the harbor. Franky builds a boat, but Sanji's group is nowhere to be found, neither is Brook. The leader of the thugs challenges Luffy to a one-on-one. When Zoro tells him that there is no time, Luffy tells them to leave without him. Zoro, Usopp, and Franky head in pursuit of the Sunny. Luffy defeats the thug leader after a while, who gives Luffy a ship. Luffy finds Brook and the others and sails towards Commi Island as well.
Meanwhile, a band of pirates are celebrating. The pirates are all fishmen. They give a toast to their most valuable member, Nami, who has finally returned with a horde of treasure. Nami walks up to the leader, Arlong, and gives him some of the treasure.
Meanwhile, Zoro falls overboard, leading him to swim up into Arlong Park. However, he is imprisoned, but later rescued by Nami and (unwittingly) Hachi. Nami orders him to leave the island or else.
At the same time, Luffy's group fights a giant sea cow before arriving at Commi Island. Also, Usopp and Franky meet a woman named Nojiko, who lives in oppression because of the Arlong Pirates. Meanwhile, Arlong meets with the Marine, Nezumi, who has agreed to not pursue him in exchange for money. Arlong promises Nezumi 100 Million berries.
Luffy's group crashes into the island while Usopp chances upon Arlong Park. Worriedly, he enters disguised as Sogeking, but his guise doesn't help him, as he is quickly defeated by Arlong. In the meantime, Arlong offers to Nami to kill Usopp herself. She walks over to him and seemingly stabs him, causing him to the fall to the water.
Later, the Strawhats (minus Nami, Usopp, and Franky) all meet up in the woods, but are confronted by Nami. Luffy asks her why she left but she answers by saying that she killed Usopp and she will do the same to the others. Luffy angrily punches Nami, causing her to fall to the ground. She rebutts by using the Clima-Tact and created a cloud behind Luffy. This enrages him even more, because she is using the weapon Usopp gave her. She replies by using Thunder Lance Tempo, although it has no effect on Luffy. He charges with a Gum-Gum Bazooka, but is kicked by Sanji before connecting. Sanji grows angry at Luffy, but Nami laughs and leaves telling the crew that it is none of their business. Later that night, Nami heads to her house and speaks to Nojiko, who she reveals is her sister. Nojiko comments on the Straw Hats, but Nami brushes it off. Nami takes off her glove, revealing a stab wound.
The next day, the rest of the crew, (minus the dead Usopp and absent Franky) are resting in the woods, when they are met with a ghost, who reveals himself as none other than Usopp! Who isn't dead. Franky is right behind him. Franky says he found Usopp near the Sunny. Usopp then relays the information Nojiko gave him, about her haunting past. Cue the flashback.
In present time, Nezumi and his Marines arrive at Nami's house (per Arlong's tip) and defile her garden in search for the treasure she had saved up. Angrily, she uses Thunderbolt Tempo on them, but they still escape with the money. Breaking down, she hears a commotion nearby. Nojiko, who saw everything, rallies the people of Cocoyashi to stand up against Arlong. Nami puts on a brave face and smiles, saying that she could get the money back. However, Genzo sees right through this and tells her that it is no longer her burden. She pulls out the Clima-Tact in anger, ready to attack the villagers, but Genzo simply grabs it and hugs her, leading to everyone rallying past her. In complete desperation, she pulls out the knife she stabbed her hand with and starts stabbing her Arlong tattoo (albeit much more dramatic in the movie :P) but is stopped by Luffy. She yells at him to leave, but she soon breaks down, begging him to help her. He simply puts his straw hat on her head in response, shocking her. Luffy walks up to the others and begins the march on Arlong Park (only twice as cool with twice as many people) Luffy calls out Arlong, beginning the battles. Luffy is attacked by Mohmoo, who Luffy dispatches, easily, but gets himself stuck. Arlong uses this advantage to toss him into the water. Chopper tries to save him but… :p. Zoro and Sanji try to save them, but Hachi, Kuroobi and Chuu prevent them. Meanwhile, Franky, Nojiko, and Genzo attempt to save them elsewhere. Zoro vs Hachi begins. Zoro attempts to use Shuusui, but he drops it, leaving him with only two swords against Hachi's six. Brook offers his sword to Zoro, who originally has difficulty with the Fencing style, but comes to terms with it, using Asura to outnumber Hachi in swords. Meanwhile, Usopp runs from Chew, but eventually turns to Sogeking and effortlessly defeats him. Sanji has trouble with Kuroobi and tries to save Luffy and Chopper, but is nearly defeated by Kuroobi in the water. Sanji manages to escape the water as Franky and his group saves Luffy and Chopper. Sanji lands a devastating combo on Kuroobi, but eventually is overpowered. Unable to stall long enough, Sanji gets aid from Robin, who holds Kuroobi back while Sanji uses Diable Jambe and finishes off Kuroobi. Nami arrives and tries to fight Arlong herself, even using the Clima Tact, but is defeated. However, Luffy arrives in the nick of time. Luffy vs Arlong, generally the same at the start, but Luffy has to go Gear Second when Arlong gets Kiribachi. Arlong can barely keep up with Luffy's speed, but manages to severely injure him. Arlong calls Nami his crewmate forever, butLuffy won't allow it. He jumps into the air and uses Hone Fuusen to go Gear Third. He finishes the fight with Gum-Gum Gigant Bazooka, destroying Arlong Park in the process. Nami breaks down in happiness as Luffy tells her that she is his friend, no matter what. Movie ends with Franky setting sail and Nami jumping onto the Sunny. However, they are pursued by Nezumi but Nami responds with a Thunderbolt Tempo, sinking his ship. The End.
Yeah, some major creative changes, but it would all make sense, IMO. 
