Back before timeskip was obvious, my main argument for Hancock joining the crew was that the crew (or Luffy at least) needed a haki teacher. Now that timeskip has come and gone, and Rayleight has filled the slot of haki teacher. This reduces the usefullness of Boa to comic relief. Or in other words, I admit there is no purpose left for Boa anymore as a central character, and I agree she won't and SHOULD NOT join the crew.
Perona…Perona, well, pre-timeskip Perona was a whiny little bitc-. Now she seems prettier and more mature, so I could in theory stand her. However that isn't really a problem anymore.
I don't have any really good candidates for Strawhat crew right now. Maybe fishman island could present an opportunity, since they are fairly exotic...things in the One Piece -world. And the Strawhat crew is very exotic indeed.
However, I very much feel like the crew is very, very complete right now. The crew is united with a purpose, they are one big family I would say. Can anyone new REALLY make such bonds with each other that we have made with current characters? If someone doesn't join on the fishman island, I'd say the chances of someone new joining will drop significantly with each passing chapter.
I love each and every Strawhat, it will be very difficult for someone to join the crew the more time passes (in relation to how close to end the manga is).