There's a lot of discussion of new newkama, or crewmates, in the New World Saga (second half of OP). Currently the main characters under discussion are Kinemon, Momonosuke, and Jinbe. I believe that these characters will probably end up joining the Straw Hats, not as crewmates, but as formal allies. Currently, the Heart Pirates are the Straw Hats' only formal allies but this is probably how the New World operates. We saw at Marineford that Whitebeard had a plethora of New World allies, each with their own crew. This was part of what made the Whitebeard Pirates such a threat to the Marines/WG. At the same time, these crews were not regarded as part of Whitebeard's family of crewmates. I think it would be hard to be regarded as part of Luffy's nakama if you don't have that family aspect; and it seems that only those who were separated at Saobody could really understand each other on that familial level. This is probably the largest obstruction to becoming a member of the crew because they do not understand what the first 9 went through at Saobody or their reason for going into hiding for two years.
I think that after the Straw Hats defeat the Doflamingo Pirates (or at least drive them out of Dressrosa) they will gain a multitude of pirates and people who want to pledge their allegiance to Luffy for winning the gladiator tournament. With this influx of recruits the Straw Hats will have to add them as allies not crewmates. Allies would not ride aboard the Sunny and instead take their own ships as a part of a fleet. Also, Kinemon, Kanjuro, and the other samurai will join as formal allies led by Kinemon, much like the Heart Pirates. I will admit that I think Momonosuke wil become a Straw Hat as the cabin boy and Zoro's trainee. Now you might say, "that's okay, but what about Jinbe? Luffy asked him to join and he agreed." While it is true that Jinbe agreed to join the Straw Hats later, he also has to consider his role as captain of the Sun Pirates. Therefore, I think Jinbe will do the same thing as Law and Kinemon and join just as an official ally. Jinbe will probably be more submissive to Luffy than Law or Kinemon but still hold direct command over his own crew. These crews (the samurai, Heart Pirates, Dressrosa recruits, and Sun Pirates) will travel with the Straw Hats as a larger fleet. As the Straw Hat alliance travels from Dressrosa forward they will form more formal alliances with powerful pirates in the New World.
I think the Straw Hats will add allies instead of crewmates because they need numbers to take on Big Mom and Kaido. Adding just individuals like Kinemon or Jinbe would not give them enough strength to take on a Yonko just by sheer size of their crew. Consider how large Whitebeard's crew was and the number of allied crews he had with them. By conquering territories and gaining allies with crews, Luffy and company would at least have shot at facing the Yonko. Also, having other crews with them would allow the Straw Hats to compete with the Hawkins-Kid-On Air alliance. An alliance, with or without Dressrosa gladiators, would give them crews to take on each part of Kid's alliance (Straw Hats vs Kid Pirates, Heart Pirates vs Hawkins Pirates, and Hawkins Pirates vs. samurai and Sun Pirates, or some variation of the lot). This would also mean that the Straw Hats would have 3 Supernovas, a former Shichibukai (of at least Supernova level), and Kinemon who is just under Supernova level strength along with Sanji. Allies of high quality (strength) + a large enough quantity (crews not individuals) = Yonko-threatning alliance.