For me it's Vivi imo.
Nobody in Wano cuts it for me. The scabbards are super cool, but Oda needs to hint at one of them being more important than others at some point, and he isn't doing it. They are the crew that once lost their captain to Kaido, whoever is going to join the SHs needs to show more hope and potential than the rest. Never mind the other issue is that they overlap too much with Zoro. And they also waited years to see their country rebuilt they might actually want to live in a prosper Wano for a change.
The most interesting one of them is Kawamatsu, but on top of already being a swordsman he also overlaps with Jimbei too much. Meh.
Only hope for them is if Denjiro turns out to be much more interesting. Assuming he's not just Zoro's master.
Carrot had hope in WCI but then she disappeared. She isn't joining if Oda doesn't think she's important in the arc she is supposed to join.
I was in Team Momo a few weeks ago but after the recent flashback it feels extremely important that the next shogun actually be there for his people, not go in another selfish adventure. So he stays.
Tama doesn't have that restriction and she has a cooler power but being a little kid is such a deal breaker. It's easy to point at Roger's crew when this is not Roger's story we are following, we are following the SHs and I need to actually believe they can knock down some marines when needed because it will be needed. Granted her hax power could win some fights but all strawhats actually have basic fighting skills and moves that can always be used, her hax will become very limited once we are out of the Zoan Saga.
After Wano ends it's hard to introduce and develop someone to be a SH considering the crew's main fight before becoming yonkou already happened. Like, whoever joins next doesn't feel like they were really part of it.
But Vivi fixes that problem because she was part of the SHs history. She is a SH in every way. And if Imu decides to arrest or try to kill her or invade her kingdom she might have no choice but to stay with the SHs for an indefinite time. At least until the manga ends. Then maybe if the crew still exists after that she will leave them again so this is a bit of cheating, but she was a member of the crew that made Luffy PK and part of the 11 pirates who fought the 11 Blackbeards, so that works for me. This plot also gives her a dream: she needs to save her island from the WG, or create a new world order where her and other kingdoms can be free. And this ties well with the plot of the SHs eventually taking down the WG.
Main issue is that we have no indication she has enough strength, but this is an easier gap to fill than for Momo or Tama. Vivi was a Baroque Works numbered agent when we first met her. She can fight, probably better than Nami or Usopp could at the same point of the story. She needs a boost to at least get to Nami/Usopp's new world level but in those two years she could have done some training, or maybe eaten a DF. It can be solved.
