If Carrot doesn't join
I'm split 50/50 between Jinbe being the last and Vivi rejoining
I don't see Oda inventing a new character to develop to join the crew, not at this state of the story
And out of existing characters, only Vivi truly makes sense. Have a very strong bond with the core members of the crew, treated differently than other princesses be it by Oda or in extra-material (
There are two existing arguments against Vivi though :
1)No role
2)Lack of strenght
But Oda can fix that
1)She can be crew's diplomat. Like archeologist it is not a historical job in a pirate crew but like that role I think it could end up being relevant in the story, during he final war involving the world, I could see Vivi vouching for Luffy and using her prestige/legitimacy to push other kingdoms to join the straw hats side
2)"how the heck is vivi supposed to fght one of Blackbeard captains". Well, Oda showed with Nami and Zeus that he's not afraid to hand his protagonists pretty generous power boost if needed. A powerful devil fruit can do the trick. Viv doesn't have any 1 vs 1, changing her fighting style (peacock) wouldn't ruin the character. Nami went fighting with a bo staff in EB to becoming a pseudo mage.