Luffy: He'd probably be a boxer or a martial arts instructor.
Zoro: A butcher.
Nami: A historian or maybe a meteorologist.
Usopp: An inventor, or maybe a blacksmith.
Sanji: A CHEF! (duh.)
Chopper: A zoo exhibit. XD
Robin: A prost- no, wait. Let's leave my perverted thoughts out of this. She would be an archaeologist, obviously.
And the non-Straw Hats?
Lucci: A serial killer.
Franky: Luffy's rival in boxing.
Crocodile: A politician. (?!)
Spandam: Somebody's bitch.
Buggy: A clown, duh.
Eneru: A church minister, teaching the Religion of Eneru to people. (Which, incidentally, I'm the prophet of.)
Shanks: An explorer.
Morgan: A lumberjack.
(I'm just damn happy none of this actually happened.)