So if I remember correctly from my previous talk about whether there would be any new members after Jimbei joined, Robby (which Sandman backed up with a link from where it came from) told me how Oda claimed that two members would join in quick succession. So once Zou is finished Luffy and some of the others will be going to the tea party, which many are guessing that Jimbei will be there as well. So I'm wondering how this will play out and what this means for the new members joining.
If Oda was a troll I could see it being Jimbei joining with Sanji rejoining quickly afterwards, or vica versa. I don't remember if Oda said new or not, and I have doubts this would actually happen. Still, something that I guess is a very small possibility if Oda didn't specify the members joining being New. This should probably just go into dumb predictions.
Going with the likely scenario of Jimbei and someone else joining quickly together, the question comes in how will this work. We could have Jimbei join in the upcoming arc where they go to rescue Sanji, which if that is the case it would seem that the other character would be introduced that arc as well. That said I have doubts that Jimbei will be able to join, because I don't see Big Mom just allowing Jimbei to leave under her services like that. The scenario I could see happening is Jimbei goes with the crew with the promise to help Big Mom get rid of Kaido by helping the Straw Hats, and in that case could be unofficially part of the crew. The big thing is it may not count as him being part of the crew and he might not officially join until the confrontation with Big Mom. This goes into my other theory.
Whether Jimbei goes with the Straw Hats or not, there is a chance that someone joins during the Wano arc. This person gets all the development like the others before, and at the end of the arc joins the crew. In this case they would go back to Big Mom who they would have to go against next. At this point I could see Jimbei cutting ties because of what Luffy accomplished and know that he could join him and just be fine. This would allow Jimbei to join the crew early on and we would have the two members in quick succession.
Of course all of this hinges on Oda's idea of quick succession and if it actually stays like that. There have times (or at least one when it came to him claiming a big name would be making a move coming much later) in which Oda has said something but it doesn't end up being filled. He can claim it will be in quick succession, but who knows what changes he might make. Either way, I'm interested in playing out the possibilities of how this will go down.
As for the New Crewmate, I can say with confidence that nobody that's in the current arc are really material for debate, so I'm just waiting for a new arc with new characters to make any predictions on that.