As we all know, Luffy has recruited 8 crew members. He said he wanted 10 at the beginning of the series. If we do or not count the Thousand Sunny, the Straw Hat Pirates have currently 9 or 10 crewmembers, including him. I have noticed anyway that they are in need of some crewmembers, anyway. Some of my ideas are......
1. A maid. The ship is definitely going to need to have some cleaning up now and then, with things happening like the zombies of thriller bark trashing the ship, and the dishes that need to be cleaned after big feasts that Sanji makes now and then.
2. An artist. The ship could use some art now and then, and it's been hinted that Luffy wants a statue on the ship, and an artist would be perfect, especially if they could make statues.
3. A scientist. Though this may be a bit redundant, because Franky himself does those kinds of things with cyborgs and such, but what happens if Franky malfunctions? If his wiring goes wrong, or his circuitry goes bad, someones gonna have to put him back together.
4. A flora/fauna expert. The Straw Hats are going to come across some pretty wicked animals and plants, some of which they're going to want to eat, and if any of them are poisonous, that's going to be a problem. They're going to need someone who can tell what to eat and what not to eat, and tell them what can help them on their journeys, such as the bird from Sky Island. They would also be able to tame animals, and a lot better than Moji.
5. A hunter/fisherman. If, somehow, there is a low amount of food, they're going to need someone who is an expert in catching food. If Zoro, Usopp, Luffy, Chopper, and Sanji can't catch any food somehow or don't know where to look, a pro is what they'd need.
6. A lookout. Someone who is always alert, and will notice a pin drop from a mile away. It's the kind of person that is always wary of their surroundings, and looks for possible exits in any room or enclosed area.
Those are my suggestions for people they may need, but people they want to recruit just cuz is another story. The limits to that are endless. Please tell me any suggestions you have!!!