I don't know who will or will not join for sure, but based on the other mates, I believe that there are some basic rules in order to join the SH's.
The new crew member usually joins at the end of an arc or an adventure and it's not always the most predictable character who joins (but based on that, it could be anyone…)
They all have strange behaviors: Usopp is a coward, Namy is a greedy bitch, Sandy a sex maniac, Brooke is… Dead... And each one of them is somehow, dumb, fun and original from one another …
They all have a dream related to Luffy's achievement (otherwise there is no real point in joining the crew)
They all have a function in the crew: First mate, doctor, lookout, carpenter, archeologist…
But most of all, they all have a high respect for friendship
Of course, there are exceptions that prove the rules, but they match for the most part