OK, I made a list.
1st: I want a HOT female devil fruit user, with high fighting strength, like strong enough to be a main fighter. I don't know what kinda ability, but preferably not a logia, and preferably an ability that can be used in battle. As far as the story is concerned, it doesn't look likely that such a character would join the Straw Hats soon, but that's what I feel the Straw Hats need now. Also she should be intelligent but fun and not serious or easily annoyed like Nami. She should be young, like maybe 19-24. I dunno what her duty aboard the ship should be, but it'd be fine if she was just a pure fighter like Zoro and Luffy. Also, if her fighting style was based of weapons or martial arts, in addition to her devil fruit ability, it'd be cool too. And since the Straw Hat's will be in the New World, if such a character were to be introduced, it'd be cool if she had Haki.
I also want the Straw Hats to have lots of "different" or "eccentric" members. Think about it, they have a living skeleton, a reindeer-man and a cyborg. So, I want them to have more people from different races, like a mermaid, a fishman, a Sky Islander (with wings), maybe even a Long Arm tribesman or a dwarf. In chapter 501, on the list of auction prices, it lists races such as minkmen, long-legs and snake-necks. Maybe it would be going too far if the Straw Hats had people from every race, but basically my point is I want them to have an extremely diverse and colourful crew. If the Sunny were bigger I'd even wish for them to recruit a giant from Elbaf.
Even though I said the first new Nakama I want is a hot devil fruit using fighter woman, it's most likely the next member will be a fishman and/or a mermaid, and fishman will be likelier, since if they get a mermaid, he/she'll likely be young like most of the crew, and mermaids under 35 don't have legs for on land. Franky'll have to build some kinda mode of terrestrial transportation for him/her if that's the case. So yeah, probably a fishman first. I hope he/she's a strong one, and as for the species, how about Japanese Fighting Fish, or Koi, or Dolphin, or... man, so many good marine species.
And as for what crew members they need, I really can't think of any. They have a navigator, a doctor, a cook, a shipwright, the works. I'm not familiar with sailing and duties aboard a sailing vessel, but this website gives a list (though I imagine there must be more positions than this):
http://www.thepirateking.com/historical/ship_roles.htm
Most of those roles are fulfilled by one or more members of the Straw Hat crew, such that a crew member may in fact hold more than one position, even if officially they only have one title. For example, Franky is the shipwright, but going by this website, he fulfills the roles of boatswain, carpenter, master gunner and more. So in that respect, the Straw Hats already have all the main sailing roles covered.
I predict the Straw Hats will end up with around 20 crewmembers, given the size of the Sunny, and the specialised skill of each member, and I like that. I think the New Age will see a lot of small, powerful crews, like the Straw Hats, the Blackbeard Pirates, and many of the Supernova crews.