Luffy originally planned to find at least 10 nakama to join his ship (based from ch.1). Including himself as the captain, a swordsman, a navigator, a sniper, a cook, a doctor, an archeologist and a shipwright compose his group of 8. Brook, the afro skeleton is a possible future member since he's a musician and Luffy wanted a musician from the very start, apparently for some entertainment.
The Strawhats are distinct pirates coz' even small in number, there is diversity among themselves. Luffy can pick whatever he wants-a magician, a pet parrot, a DF user, a clown or an okama, to fill the 10th spot, but, he may also not get one.
Because their next destination would be Fishman Island according to Kokoro (after the current Gecko Moria/Florian Triangle arc), they will absolutely encounter mermen in that place. We know that Luffy can become capricious on any occasion and a thought of getting a recruit is possible, just like in the case of Brook. Now, getting a merman crew member would sound cool coz' they get a member that can scout and attack from underwater. And they are powerful. Another Arlong (good guy) would complete the picture I'm thinking.
It's just a speculation.