1. Current active Straw Hats include 
Luffy, Zoro, Sanji, Jinbei, Nami, Usopp, Robin, Franky, and Chopper= 9 People. For the purposes of this thread, 
I do not consider Vivi a Straw Hat (the reason being even if she joins later I do not think anyone sees her fighting other crews).
2. Considering Oda's statement was verbatim 
"about 10 people" and considering that in the very first episode, Luffy said that he wants 10 people for his crew and considering that One Piece is nearing its end, one theory is that 
one more will join. (Even though I am fond of the parallel of 4 joining in each major chapter of the story: East Blue, Paradise and New World). 
3. Considering that 
balance is important, at the moment the crew has:
- Three Devil Fruit Users: Luffy, Robin, and Chopper (1044 to clarify, Paramecia and Zoan)
 
- Seven Men and two Female
 
The next Straw Hat, the 10th that would strike this balance could be a 
Female Devil Fruit user. Ideally, the devil fruit should be 
Logia which is missing from the force of the Straw Hats.
If Luffy's Devil Fruit is finally a Mythical Zoan, Yamato's chances of joining might drop significantly even if 
1. she does meet the criteria of female devil fruit user and 
2. she is the perfect match for Catarina Devon in the final battle and 
3. she has an adequate background for joining the Straw Hats.
Thus, for the
 FINAL Straw Hat, 
Yamato could be joining however, Oda might have in store a surprise meeting the full 
Female Logia Devil Fruit user criteria in the next arc. 
(Or personal opinion: let them both join!).