And back to my theory of Isshin being Kenpachi during time shown in current arc.
There are few facts we are sure:
1) Isshin is Shinigami. This one is 100%, we've all seen him.
2) Isshin knows Uruhara. While this one is also sure we can't assume too much based on this fact as we do not know when they've met for the first time, still narration laws suggest they were already aquainted before leaving SS.
3) Zaraki killed former 11th Divsion captain thus taking his place and achieving title of Kenpachi.
4) At Uruhara captainship ceremony there was no Kenpachi from his time ergo it could be anyone.
5) We do not know exact time when Zaraki became captain but we can exclude him being the Kenpachi in question, as Tousen was already around when Zaraki killed 11th Captain.
Above are the solid facts. And now to subtle hints/persumptions.
1) Isshin was a captain. Never stated, but many people are having high hopes due to armcloth and his talk with Grand Fisher. I give it about 70 to 80%. Why? Universal rules of narration.
2) Isshin left SS around same time as Uruhara. While first part is virtually 100% sure the other is not. Still beat-up shonens are almost entirely cliche based and this fits the general rule.
3) Captains rotation is about same as in big lawyers corporations in NY.
While I have no proof to back up my theory in ways of it being sure I think that points above are enough to prove it possible! Considering it would be one of those shocking events (you know, like revealing that Itachi was good etc.) it is more than probable that Isshin was Kenpachi in time, in which current arc is taking place. If I am right we'll know soon enough. Well, same is if I am not right but hell with it.