Dressrosa has a ridiculous amount of new characters and a lot of them are in some way indebted to Luffy or the Straw Hat crew. Obviously, due to this people think that one of them will be a new crew member. Mainly poeple like Barty but i am against this and think that no one will join because if one joins the others will too and then the crew would be WAY too big. I could see the straw hats sneaking away from some of them to get them off their backs similarly to skypia but without the treasure.
So with that in mind i am willing to bet that it will turn into a commander/allaince type thing with all the respective crews that Luffy has made friends with. Chinjao has already offered for Luffy to take leadership of the Happo Navy and Barty would probs do the same with his crew. Law is the one i wonder about the most because he has said he wants to find the One Piece himself but would he give up his dream to help Luffy after Luffy kills Mingo, who Law has wanted to kill for the majority of his life? It just doesn't seem likley that Luffy just leaves him to his own devices and they become enemies again. Though he did the same with Coby in a way.
The only problem i see with this is that Luffy seems like the kind of guy that wouldn't do something like that with the other crews, besides Law. Over the entire series he has left friends behind constantly. Whether they are pirates, criminals, or just ordinary citizens. So will he set it up that way or will the other crews just do it without really asking? Or will someone that has some sense on the crew make Luffy agree to it after it is offered?
These crews wouldn't necesarily be commanders but they would still fly the SH flag under theirs similar to Bellamy flying Mingo's flag. This would also help to expand his influence in the New World and possibly make them one of the more powerful groups in the New World and help Luffy on his way to defeat the yonko that he has to or even become one. He has already started to do this with Fishman island and will probably do the same to Dressrosa and other islands so, is it really that far fetched that he starts amassing crews under him?
Another thing to point out is that even though Roger was the Pirate King and characters who have met him, and consider him to be a friend have been introduced, none of them have been shown to be commanders of somesort under Roger, though none of them have been pirates, if i recall correctly. So did Roger do it with commanders or did he just set out with a crew and get that strong? I somewhat doubt that a group of 10-11 people will be able to conquer the New World by themselves and it seemed like Roger had a large crew.
All in all i think that he will and that Barty, Cavendish, the Happo Navy, and Law will become allies. What do you think? This sorta just ended up in me spewing ideas on top of each other but do you think that Luffy will adopt this system and who all will join in on it?