I would want a character who can do minimum one of these three things.
- Bring in a new perspective that will be outside the Straw Hat's, to create discussion and conflict between the protagonists.
- Bring in a character who has an existing-good relationship with Luffy, Zoro or Sanji, so we do not need to waste time establishing ties.
- Bring in a character who has an ability that genuinely would change the arc going forward.
So I would pick Boa Hancock because she scratches all three boxes.
She brings Luffy up to speed on Reverie a bit more and on the events of her island, namely Blackbeard, Teach and Koby.
She has pretty unique powers that every antagonist would need to plan around or be stoned by.
Boa also has deep ties to the Tenryuubito and this arc happens to feature the 3rd highest rung of Tenryuubito.