Okay so we all say to ourselves when we watch or read One Piece "If they had THIS… they could..." etc...
So what do certain One Piece characters "need". This could be a physical object, weapon, tool, information. This could be a trait that they desire, or an emotion.
I figure that Luffy needs a weapon that will allow him to combat Logia type users (IE Crocodile, Smoker, Aokiji, etc) that can make their bodies impervious to damage.
So Luffy will get a set of Seastone gemed Bronze Knuckles. The Seastone gems are imbedded above the Bronze, so they won't touch him directly, similiar to Smoker's Jitte. This will allow Luffy to deliever his important punches without having to Bleed to death (Crocodile), or having to wait for a breeze to come by (Smoker), or being on fire? (Aokiji).
This is just my example, feel free to throw in your own piece.