Personally, I feel like we have enough Straw Hats as is...
But here's my theory on where Oda's taking the crew:
Firstly, there are general commonalities that all the Straw Hats share:
-They have a dream that (for them) requires their presence on the Pirate King's crew
-They exceed in some profession
-They have been orphaned or have lost their family (figuratively or literally)
-They have complete faith in Luffy
It's important to note that the characters that join the crew do not have to start with all these traits. For example, Nami did not initially have faith in Luffy and the crew, so she only pretended to be a member. When she truly believed that Luffy could save her, she officially became a Straw Hat.
Yamato
Very likely to join imo.
After Kaido and the Beast Pirates are wrecked, Yamato will functionally have no home. There is a 0% chance that he will take over the Beast Pirates or become the Shogun of Wano.
The people of Wano would never accept the son of Kaido as their new ruler when Momonosuke is still alive and Hiyori is of age to rule... and with the way Oda has been writing the subplot of gender equality in Wano, I think he's setting up Hiyori to take power.
Yamato also wants to be free from the literal AND metaphorical shackles (remember this is Oda there's always subtext) that he was subjected to for the past 20 years... and remaining with the crew to uphold the lineage of his imprisoner doesn't seem to be what this character would do based on how they were introduced and have been written until now.
Since his introduction, Yamato has been giving an unwarranted amount of significance considering his late introduction in the arc, and he's been written with a purpose that goes beyond Wano. His final "chains" to Wano are with Momo now in the form of Oden's journal, and he's tied to Luffy through Ace.
Law
Almost certain to join.
Until recently I thought everyone that thought Law was gonna join was smoothbrained... but after the 2 most recent chapters I've done a 180 on that opinion. He meets all the basic conditions.
He's an orphan, easy check. He even lost Rosinante and he was an enemy of the Doflamingo Family.
People argue that he'd have no role on the crew because Chopper's already a doctor... but Law wouldn't be the doctor of the crew. People trip up too much about his epithet "Surgeon of Death" as having to do with something about doctoring, but Law's character and motivations have nothing to do with the medical field, apart from stabbing Mingo straight in the heart. His role on the crew when he joins will be the same as the role he's had since he's allied himself with the crew - he's the Tactician. I buy into the theory that Law is somewhat based on Napoleon due to the name (Water Law - Waterloo... like Oda has admitted to doing with Roronoa and l'Olonnais) and he was also known to be a great strategist. His devil fruit is literally perfect for this, he can reposition people in a flash.
We recently found out his dream in chapter 999, he wants to discover the meaning of the Will of D. I believe that this secret is tired to the One Piece, and that the only way for him to discover this secret is to be on the Pirate King's crew.
Some people will say that this is disrespecting Law's character and status as a rival to Luffy, but consider how Jinbe's dialogue has been carried out in recent chapters. There is no shame in being a member of the Pirate King's crew... in fact there should be no higher honor. We've been told that Luffy's true power is to rally people to his side. The true Pirate King will have no rivals - he would be singularly the strongest and be able to conquer any foe. But Luffy is a king that mainly conquers with friendship. It should come as no surprise that the man with the future WSS on his crew (who many have also argued is a rival to Luffy <.<) and a former Warlord, would be able to recruit another former warlord and rival. I know a lot of us may not want this outcome for Law because we might think it demeans his status... but it's clear that to Oda, making someone a crewmate is a promotion.
And after chapter 1K, where Law wordlessly followed Luffy's orders, I'm pretty convinced. After this ark, the whole world will recognize that Luffy is on track to being the future PK, including Law.
Smoker
I don't think he meets any of the criteria. I don't see how his character motivation his uniquely tied to being on the PK's crew. If anything I think he'd join SWORD.
Carrot
I'm not so sure about her joining.
I don't have much to say about Carrot that hasn't already been said... a lot.. but Oda is without a doubt setting this character up for.. something? If I interpret her dream/ambition in relation to how she would fit on the crew in the most generous manner, it would be like this: She wants to see the "Dawn" of the world that the minks have been waiting on for generations, and that her mentor lost his life to secure, and she believes that the PK will bring this Dawn. It's possible that Carrot, who was written to be unique amongst the non-scabbard minks in that she served both dukes, will represent the mink tribe as their witness to this event. Her role on the crew would be scout (not lookout). Upon their introduction the minks are shown to excel at stealth - to the point where Luffy could not sense them with his guard down even though he had passive CoO at this point - and carrot was in particular highlighted to excel at speed.
But honestly it could go either way for me... she could be a red herring. It really depends on where Oda takes the story with the minks as a whole from this point, and how the fight with Perospero ends.
Momo & Tama
These two are a package deal - either both join or neither do.
Oda has consistently written these two as a duo since they met - in chapter 999 their reactions were literally synced.
I'm not sure how Oda would work these 2 into the endgame of the series considering how we're gonna be having bigger fights from here on out... but Roger had kids on his crew ¯\_(ツ)_/¯. If either of them join, then there's no reason why the other wouldn't. If Momo joins because of some desire to see what his father saw or whatever, then they have a kid on the crew and there's no good reason for Tama not to come along. If Tama joins because Luffy wants to fulfill Ace's promise, then there's no reason Momo wouldn't tag along if he wanted to. He was born on a pirate ship lol. At this point it seems Kine'mon has complete faith in Luffy's competence so I doubt he'd be the one to stop Momo, and he's too young to be Shogun right now anyway.
But Oda could always just stick with the "no kids allowed" thing, its really a toss up at this point imo.
Thanks for reading lmk if you think I'm full of it