It's very hard for me to see Crocodile joining. I don't know why, but it just doesn't fit to me. Idealistically, there will be another female character, since it seems a bit too testosterone heavy on the ship, but then again, this is a shonen manga. As for the whole "boss" thing, I'm unsure about how to properly interpret that.
I mean "boss" could mean any kind of villain really, if you think about it. I guess I think about the games when I think of "a former boss" joining. I mean, there were the big bosses, but there were also minor ones, which I think should also be up for consideration. Honestly though, there is a while to go before the crew goes to the New World, unless, of course, something magically happens, and they're all reunited. It seems like now, Luffy will put going to Fishman Island on the back burner and finding the rest of his crew first. So there's time for new characters and "bosses" to appear and then somehow join the crew. Maybe. Just throwing that out there.