I thought a lot about a possible movie series of One Piece in the past. The way I see it, the movies either could be the ne plus ultra or a big load of shit. But, if a competent director (and hopefully, a fan of the mangas itself) would work on it permanentely and at long sight, it COULD work - although I have no idea what director could fulfill my ideas. Offhanded, I think of James Mangold or Alfonso Cuarón. But we shouldn't rush on that, the right director probably is by far the most important point to argue about in the first place.
But before I lose myself in thoughts about that, I'd like to tell about the movies itself. As I said, it should be a well planned series of movies, with cross references in the story (like the OP story itself and so on). The first flick should focus on Luffy, his childhood, the departure from his home town and his first adventures - for that, I aim for a 1:1 adaption of the books, in word and picture, but the whole plot about Luffy's past shouldn't take much longer than 30 minutes.
After the viewer met and learned to love Luffy for his character, the story concludes from Alvida (who will be finished off very quickly, like ten minutes) to Morgan. Luffy and Coby will meet Zoro, defeat Morgan and team-up - just imagine the first time when Zoro is seen with three swords! Sure, it would look unfamiliar in real-life, and maybe it would be a problem to depict it - but it wouldn't be impossible.
After Morgan is defeated and Coby joined the marine (at this point, the movie should be about 50 or 60 minutes). Now, after everything is ready for the "real" story, the movie prepares itself for a big climax. Luffy and Zoro meet Nami, Poodle, the villagers and, of course, Buggy and his companions are introduced. Buggy would look rather grotesque with his big red nose, but like Zoro's Three-Sword-Style, it isn't impossible to depict. And after all, the viewer should notice it's a "comic movie".
After the story continues about 30 or 40 minutes (and hopefully doesn't get boring in this time, the time in the village has a little deadbeat there in my opinion), the grande finale starts: While Luffy totally whipes the floor with Mohji and Richie, Zoro has an astonishing fight with Cabaji that definitely should take a few minutes. At last, Luffy and Buggy face each other. They fight (more serious, not so comedic as in the manga, although the dynamic in the fight totally works), have a little stand-off and Buggy reveals his past with Shanks, and hopefully, the viewer would have his first WTF? moment. The fight continues, and with Nami's help, Buggy loses his body parts and is send into the sky by Luffy.
At this point, the movie ran about 110 or 120 minutes. That's neither too long, I think, nor too short for a first, epic movie that is starting a great movie series. The movie ends with Luffy and Zoro accompanying with Nami and depature for the sea - a movie with a finished plot, but a possibility for many many sequels.
The second movie would contain Kuro and Krieg, and would also introduce Usopp and Sanji, of course (hence the most important crew members for the next movies), Arlong would get the third movie for himself, but the flick would end the same way as volume 11 so the fourth movie could start with the entrance of the Grand Line.
The title stories (what's happening with Coby, how Buggy came to Loguetown with Alvida) would be told in freeze images while the credits roll (maybe in gigantic pictures drawn by Oda himself?). But I really haven't planned that so well, I just got an idea of the first movie. The next movies would contain many hard decisions (what about Usopp's nose, for example), but I'm going to think about that in the near future.
Of course, the first movie alone has many complex problems. The hardest would be the depiction of the characters - we've read the mangas too long to forget the faces of Luffy, Nami and Buggy, but with capable actors and intelligent costumes, it could work. Most of us have seen good cosplays (whether or not you like them, I don't like them that much myself), made by fans with low budget - now imagine an actor, casted by specialists to play the accordant character, and a great costume, made with a really high budget.
Anyone remember the topic where we discussed actors to play OP characters? There, I wrote that I totally could imagine to play Jim Broadbent Poodle, the mayor of Orange. Now please have a look at this picture of the character he is playing in the first Narnia movie. Doesn't he look a lot like Poodle? And that's just by coincidence. Imagine how well the character would look, or Zoro, or Cabaji, if they would be made with real effort.
Yeah, that would be my idea of a first movie. Please tell me what you think of it (or the concept of the movie series I aspire).