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Oda actually planned on having Thriller Bark happen before Water 7. They would have fallen from Skypeia into Thriller Bark, recruited Brook, and then had Water 7 go straight to Sabaody. I think he rearranged it so there would be some breathing room in between the two heavy World Goverment centric storylines.They could also shuffle arcs around to work better for live action. I know I personally wouldn't want that, but if it makes for a better show to, say, do Skypeia before Alabasta, then they may do it.
I know that specific example has some weird narrative implications and problems, but I was just using it as an example.
I think the way he did it in the actual story works better too. We get Thriller Bark which is horror themed, but it's all a cartoony caricature of horror, then you get put in a false sense of security when you get to Sabaody, a fun bubbly theme park, and then it smacks you in the face as the real horrors of the world exposed