Hey guys, this may sound almost impossible at first, but after connecting a few dots, it made some sense to me and i hope it makes sense to you too.
It was generally known that Oda (god) have a liking to reference his characters based on the real world characters (e.g.: Eminem, Popeye, etc) and i've already been pondering on which character would possibly match closest to Luffy. Although Oda has stated that Luffy is an original creation, I can't help but to notice his resemblence to no other than Loki Laufeyson Here's why:
1. Lame as it sounds, Laufeyson does sound similar to Luffy-san when being attemted to pronounce by a n00b english speaking Japanese though none of his crew actually calls him that. I realized this when Wiki-ing for One Piece where the pronounciation like that of 'R' and 'L' is often used to distinguish between the real characters and the ones in the world of OP (though Luffy and Laufey has often caused stumps in my train of thoughts in this matter):
2. Loki, a prominent figure in both the Norse mythology and Marvel comics is born a retarded frost giant (one that doesn't look like a frost giant at all due to his human like figure). I might have to add that "Laufey" in this case, is the name of Loki's mother which had caused another (gender) stump. This may also explain his Gear 3 Gigant based attacks.
3. I'm sure I do not have to go into the relations between Norse mythology, Vikings and Pirates right?
4. Loki is also known in the Norse mythology as the God of Mischief: I can't really think of anyone else in OP more mischievious than our beloved Monkey D Luffy himself.
My two cents ^_^