The vertex could be better. Just because there is already a vertex over someone doesn't mean it has to be over them. The best Sora is probably Miacias' over Marth, but the psa is over Pit's. I plan to start watching One Piece this weekend at one friend's constant nagging demands request; I can't really say too much in the way of help at this point. All I know at the moment is he is a rubber-like pirate.
Snake seems like a fine candidate. How is his file size? I would think it is alright considering he has a ton of different weapons already.
Yes, because luffy uses lots of weapons he needs to be over someone who has weapons >_>
Luffy is mainly a hand-to hand fighter. Other than the occasional Ship Mast and even the bow of a ship. I doubt he's used any weapons.
Pirate does not always = weapons.
If the moveset was based on the Pre-skypia archs, I would hope that there's not many 2nd or third gear. The only ones he had at that point are Gomu Gomu Baloon and Gomu Gomu Net, and he had to do those changes manually.
Marth actually has a bigger filesize limit than Snake at least according to the PSA filesize limit topic.