If anyone were to join, I'd personally love to see Bon join the crew, but that is a whim at best. Mr. 2 reappearing and joining would be akin to a *poof* here's brook type thing, but one I'd love to see happen.
I do not see Perona even having a remote chance of joining even if she and Zoro became best of buddies. Never once has Zoro ever made a request of Luffy to my knowledge, making the "she followed me home, can I keep her?" that much more unlikely.
Hancock doesn't have a chance only because if only because a bizarre love triangle of Sanji drooling over a Hancock drooling over an oblivious Luffy would derail the crew dynamic.
I would say Jinbei after two years of acting as FI's patron is the most likely if anyone at all has a chance. Relative powers of the crew are entirely up to Oda though I would imagine the monster trio at least needs to be on par with the heavy hitters like the Fishman.
Other than early in One piece Luffy talking about 10 crewmates, is there any other indicator we really can expect two more crew? I for one would not mind sticking with the crew as it is.