SPOILER :
B.A.Z. Jyuushirou and Byakuya didn't respond to Kurosaki's name but rather his resemblance to Jyuushirou's previous Lieutenant (Vice-Captain isn't a rank its a translation rather than a interpretation of Fukutaichou) : Kaien.Anyway, look at Kenpachi's coat, its ripped and worn. Look at the cloth around Isshin's upper arm, its quite possibly not able to be worn as a jacket anymore so therefore isn't worn as one.
Ichigo also proves to be destined to Eleventh division candidacy by his methods, all brute force and combat skill, a trait one can see him inheriting from his father.
I do recall it said that Zaraki killed the previous captain, and I also know the shinigami from the filler arc was of the mangaka's own character design (the one with the rainbow sword) which suggests quite strongly that he is dead.
My reason for thinking he's the previous 11th captain despite this is his relationship with Urahara. I think his powers were depressed because of the gigai, like Rukia's, and I also expect he left the Soul Society by Urahara's quick thinking. (To cite reasons why Urahara would be capable go into in manga spoilers not just anime spoilers.)
The other options are that he was replaced by Hitsugaya or Tousen (I reject Soi Fong not for Pix's reasons but for the reason that Yoruichi used to be 2nd captain.) I doubt Isshin would have left the Soul Society so that Aizen could place another loyal soldier as a Captain so I rule out Tousen. I have no reason to rule out Hitsugaya but as I prefer the hypothesis of him being the previous 11th captain, I'll stick to that.