THINGS I WANNA MENTION:
1. I think that we won't see anyone join the Strawhats for another 100 chapters (so around 850, anime around 750+). However if it does happen it will definitely be in the next arc, not this one.
2. I would really like for Jinbei to join and I think it will happen later on. Another chick would be nice for some diversity, not Vivi. Honestly, she has no use on the ship. If you recall, all of the members have a dream of their own that they want to pursue and they all have their own special abilities. Vivi is too ordinary to join with Straw Hats at this point in time, she might join after Luffy becomes Pirate King. For some reason I feel like a lot of returning characters will re-appear with the Straw Hats when Luffy becomes Pirate King. Possibly there will be a time skip in between the final arc and the post-final arc thing.
3. As for this arc, I really don't know what wil happen between Zoro and Fujitora. It's too early for Zoro to beat an admiral, though that would be an interesting twist, which would however, make Zoro OP. It will most likely get interrupted, by something even crazier.
4. I think it would be very interesting (this will sound confusing) if after Luffy beats Doflamingo, while Zoro and Fujitora are still going at it, that Kaido steps in, and after the Strawhats try to escape, Dressrosa gets destroyed or something crazy, and then we get a merging plot of a new arc, where prime focus is on the two Yonko: Kaido and Big Mom. Just throwing it out there.
IF YOU WANNA REPLY, number your points according to my list so it's easier for me to read and for others as well. Efficiency is key to me.