hmmm, yeah Rukia should replace Kaien because of various reasons.

The fact that Kaien and Rukia were practically sister/brother to each other. She finally knows here zanpakuto's name and released here shikai, its kind of like a female version o Hyorinmaru.

Then again, maybe Ikaku I mean he has a Bankai but not called a captain, well so does Ichigo. Maybe Ichigo should fill in, to my opinion, It seems as if Ichigo was the reincarnation of Kaien Shiba, almost looks alike, spiky hair, etc. So now you've heard my side of the story.