The jobs left that were typically on a ship are:
1. Sailmaker
2. Cooper (Barrelmaker)
3. Helmsman (Ship's Pilot)
4. Quartermaster
5. Diplomat
6. Log Keeper (Usually Second Mate Does this)
7. Botanist/Herbalist
Breaking it down:
1. Important, but I doubt we'll have a sailmaker in One Piece. Nami seems to be able to sew fairly well, so she can repair the sail if need be.
2. No need, there's refrigeration in One Piece. The only time I've even seen them use barrels outside of filler was to make their promise just before entering the Grand Line.
3. This is the most likely candidate. Nami is the navigator, but Chopper drives the ship. There's no telling how he'd do in short-term navigation situations. Remember in the DBF? Nami had to guide him and they still lost (Mostly due to Foxy's cheating, but still).
4. Each Strawhat has a unique fighting style. a Quartermaster wouldn't be very practical.
5. They're pirates, not emissaries. Besides, Robin usually explains anything they need to know about a new Island.
6. Nami, I think, has been keeping a log of the journey.
7. Chopper knows his plants.
So, Helmsman is the most likely candidate. A strong, 6 armed fishman (or even woman, if Octopako shows up), perhaps?