I’m becoming less and less convinced that we’ll get any more SHP before the story ends, especially if Oda’s really sticking to the 5 year plan this time. It’s way too late in the game to introduce a new character and have them organically bond/travel with the crew in time for the shit to go down, and as far as existing characters that could pull a Jinbe, I don’t see many options more viable than Law and his crew becoming subordinate to Luffy (which doesn’t exactly fit his character) or Carrot since she’s already spent significant time with the crew.
As unlikely as it is, I think it would be cool to see one of the Scabbards join them after this arc concludes (my preference is Kin’emon), but this also doesn’t seem like a real possibility if Momo becomes the shogun after Orochi falls...he’ll want his father’s trusted retainers to help him rebuild Wano. Tama could be in play, but she won’t be reliable in a fight past her apparent ability to tame Zoan users.
People have theorized for years that Smoker and/or Tashigi might join, but I think they work better as Marine “allies” than actual pirates.
Maybe another Supernova that we haven’t seen a lot of, like Bonney or Urouge? Again, not sure how that would work, but it would make the most sense out of all of these nonsensical options.