With the latest chapter Carrot’s chance of joining has increased more. Now I am going to mention the things why I think Carrot’s going to join:
1. Her unique introduction to the show (attacking Zoro).
2. Her dream to explore the sea ( we’ve seen that she didn’t even know about simple things as lighting and sea.) Carrot’s dream is to leave Zou and explore the world. She obviously could not do it by herself so the appearance of the Strawhats was the perfect opportunity for her.
3. Lookout. Carrot is going to fullfill this role on the ship. There have been several incidents where Carrot used this skill.
4. Her interaction with crew especially Chopper.
5. Fighting abilities
6. Sulong – Now in the latest chapter we have seen that even Pekoms (sorry Nazoms) who is crew member of Yonko with 300,000,000 bounty, who is far more experienced than Carrot is not able to control his Sulong form. It was stated that it’s hard to control the Sulong, you have to train hard to control it. We know that Carrot can control it and how terrified Big Mom Pirates were when they saw her transforming. So it’s pretty clear Oda gave her this specialty for a reason.
I think she is going to join the crew unofficially this arc. Obviously Straw Hats are not going to turn back to Zou to drop her. She is a strong fighter and mink are allied with Straw Hats, so she is definitely going to Wano with them. And she is going to fight someone there. IMO she might join the crew then. By then we will get more character development from her and her past will be revealed. And she will join officially in Wano. This will be same as what happened with Nami and Robin.
*Theory by GodSogeking