Not sure if this has been mentioned before so I'll state my thoughts on the next nakama. I know that certain predictions have been stated and beaten to death but until they are disproven…the beating shall commence forward...I guess forward wasn't necessary but felt like leaving it there just because.
I think the theme for this next installment of nakama will be different races/species. Before the Grand Line all four recruits of Strawhats were "regular" people. No Devil Fruit powers outside of Luffy. A swordsman, navigator, gunner, and cook. Then the Grand Line nakama were...weird. A talking bipedal doctor reindeer, mysterious able to produce multiple body parts archaeologist, a shipwright cyborg, and undead musician. All but one were DF users. The pattern of Boy, Girl, Boy, Boy was implemented on both sets of nakama.
The gender pattern could be used as one reason why Jinbe has yet to join(outside of him being "too strong"). I think he will rejoin or rather officially join during or at the end of the Big Mam arc which I suspect will be after the Kaidou arc. The Yonko are the big fish in the New World and Law's plan is to eventually take out all Yonko so potential 4 big arcs. This gives time for at least TWO nakama to join before him. So Jinbe will be the 3rd member or possible 4th if more than four join(just my hunch is all that he won't be last). So a boy will be the next nakama.
So who in this current Doflamingo arc would fit and/or help the Strawhats the most? I am leaning on Leo the dwarf. I think these little people are going to wreck havok come war time and be very strong allies. Now he fits with the different races/species for each new nakama of the new world being a little person/dwarf/whatever you want to call them. Now these lil buggers are outside of being strong and fast are great cultivators and can cultivate any plant. This alone would be a tremendous asset to the Strawhats and give them the official position of gardener. They could have their own section of plants they tend to. Here are some benefits:
1. Herbs for Chopper's medicines
2. Spices for Sanji's cooking
3. Cultivate healthier orange trees for Nami
4. Maybe be able to grow Adam Wood for the ship for Franky
5. Regular wood to make paper in books for Robin or instruments for Brook
6. Age alcohol for Zoro
7. Grow and breed Pop Green seeds/plants for Usopp
Now why I think Leo is the best suited dwarf is mainly because of his Devil Fruit the Stitch-Stitch Fruit. He can stitch ANYTHING together. He could...I don't know...stitch a broken weapon together. Maybe a precious artifact that was broken for countless years. He could make quick work fixes to a ship. And he will definitely be able to stitch big cuts to the body or missing limbs back together. This would give him the role of surgeon to Chopper's doctor. They'll be the JD and Turk of the Strawhats.