User blog comment:Fintin/Chapter 704 Review/@comment-6971349-20130404145605

Personally, I think the next crewmate is going to be whoever eats Ace's devil fruit.

This is not an absolute though, because someone could just steal it and eat it fast. (One bite is all it takes after all.)

If that theft doesn't happen, and Luffy is able to get ahold of Ace's devil fruit, he would likely try to pass it to someone who would share Ace's spirit and will. And thus that would make whoever it is a prime candidate for crew, but again refusal could still happen.