I think once Ikkaku takes on Kenpachi and feels that he did his best and feels he is on the same level as him he will become a captain, and he would be a crazy captain because he dont take no crap.
I guess Renji but he really has only won like one or two fights since he has had his ban kai. Rather sad.
The last one I think that it should be Matsumoto because I just have a feelin that she has a ban kai but is just keepin it hidden until she sees Gin again and she will beat his ass with it.
But hell thats me.