I found this bit of interesting material:
http://www.onemanga.com/Wanted_Oda_Eiichiro_Tanpenshu/5/44/
Heh, it's garp. Anyway, I think one piece will not be any of those things. I think it will be something totally worthless that no one cares about. Except, of course, roger himself. Finding it, luffy will realize that he already has his treasure, and the whole crew will give that sort of smile on their faces that's the sort of expression of eagerness to go on an adventure. Then, luffy will start back at the beginning of grand line as the pirate king, etc.