I like hunter x hunter better in than Onepiece when it comes to story lines though..
I'd like to advice to look more into FMA if you want to learn how to make a good long story, it might not be that long but you can see how everything of how stuffs connected to one another.
I mean if you haven't watched/read Onepiece yet and you'll have to start to episode/chapter 1 then I don't know when will be the time when you'll actually make your story.
I would probably agree if it's Naruto, Bleach or Onepiece or most of TVtokyo productions.I like a lot of animes mostly, I do read manga but, lets face it, anime is better ...
If you want to know why everyone disagree with this try to watch something like Black Cat or Gantz then read the manga and you'll probably see why we're so disappointed on how anime turns out