At this point, I think there are 4 candidates that make sense, and everything else is cope.
Law is the most likely from an objective standpoint. I personally still see Stussy joining because of the dream, but it's hard to deny the case for Law. He already had a perfect Strawhat joining arc in Dressrosa, he has a dream as of Wano, several roles he could fill on the crew, and he's one of the VERY few characters who could fit the Kuzan matchup. The only thing holding him back was his crew, which is now gone, and Luffy already considers him a nakama. He was even supporting Luffy after Ace's death like Jinbei. The only arguments I see against Law now are "I just don't see it" or "it doesn't feel right". There's no case to be made against him.
Bonney is also a strong choice, but with some difficulty. She's the clear frontrunner for an egghead island nakama given her narrative importance and her friendship with Luffy. That said, she could easily end up with the RA since she's Kuma's daughter, and she has no hint of a dream or role. Could easily happen.
Stussy, my personal prediction.I think she's a great dark horse candidate. She clearly has a tragic past that we'll see based on Vegapunk's words, she's a scientist (or a clone of one at the very least) which is a great strawhat role, and if she inherits Vegapunk's dream it'll be set in stone. Vegapunk's dream is not only perfect Strawhat material, but also directly related to the scientist role, which is a common theme among the strawhats, like the navigator's dream to draw a map if the world, or the archeologist's dream to find the secret history of the world. I'm fully convinced that a strawhat will have that dream, and that it won't be Vegapunk. She's also already invited to sail with the strawhats after egghead and has nowhere to go. Unlike Bonney she has no connection to the RA. The potential is all there, especially if Vegapunk dies, we'll just have to see if it pans out.
Boa Hancock is the fourth, but she has a weaker case. The setup is there for sure. She can't stay in Amazon Lily, the previous Empress just showed up to take over for her, she needs yonko protection for her country, and she can get Luffy's vivre card from Rayleigh. Everyone says "they'll join the RA" for every character they don't want to join, but that's nothing but cope for Hancock. She needs a yonko, not the RA, and you all know deep down she'd choose to go with Luffy even if the RA made more sense. It's hard to imagine what she'll do if not joining the strawhats, but she still doesn't have a dream or a role, and unlike Bonney and Stussy, she doesn't have the benefit of being in the egghead arc, and there's not much time for more arcs. You could stretch it and say that Amazon Lily was her character centric arc, but it's a little iffy, and Luffy has not defeated her personal villain like he did for every other member.
Any other candidate is cope. Smoker is just based on the Kuzan matchup which he can't possibly win, Vivi still can't become a pirate because she's still committed to Alabasta, even if she has to be on the run for now. Say what you will about your headcannon of the strawhats disbanding, but if she joins, it'll be as a temporary ally just like the first time. Carrot and Yamato both finished their stories already, and I'm not going to bother with the random cope campaigns with 1 or 2 supporters. It's one of these 4 or no one.