Here are the current clues siding with the potentiality of a new strawhat (from the lest to the most relevant):
- That funfact about Luffy's senyu saying that she wonders who will sign next
- Luffy said that he wanted 10 members and it was officialy cleared recently that he didn't included himself
- Oda naming each member after a number from 1 to 8 (Brook was named "the eight person")
- Oda opening back the Nakama storylines with Yamato
All of this is showing us that there COULD be someone else.
Here are my probabilities right now:
- Carrot still has a shot but its almost gone, there are strange things with her storyline : 5/15 %
- Vegapunk is the most likely right now but Oda needs to put up the work: 40/55%
- Smoker's characterization indicate that he is not someone meant to join the sea, rather than someone who needs to change the system from the inside: 1/5%
- An unknown character could also be a possibility : 30/40%
I don't see anyone else joining right now
Now, note that Oda could make someone join the crew at the VERY END of the story. The development would therefore come earlier. So let's stay aware of any strong development in the future.