Too many theories...
THEORY TIME! Okay, fair warning, I think this is really stretching it but I honestly think that Isshin is actually a part of the (most likely) the Royal Guard or a relative of some sort of the Spirit King and his Royal Family.
Why?
A) Isshin could have been most likely a captain (due to his cloak in shinigami form) but no one from Soul Society recognizes him which could mean he was a captain before the ones we all know and then got promoted to Zero Squad.
OR B) Isshin is a relative of some sort of the Royal Family. I could see it as Isshin somehow committed treason (but was falsely accused or just ran away) or whatever. But since the whole Aizen thing, he's been rediscovered and is now in the process of being brought back. Hence the new guy. He could possibly be one of the bodyguards for the Royal Family. But if that's true, I'm thinking that (so far) this guy is too loyal to Isshin and his family and therefore can't bring him back or some other reason.
Which brings me to my next point: the mysterious energy around Ichigo. Obviously it's from these new mysterious people. I honestly think (again, so far) are trying to protect or keep Ichigo and his sisters away from Isshin (at the moment).
DAMMIT, KUBO-SENSEI!! Why can't Ishida ever keep his arm?! And was this really necessary?! It's always Ishida. Always. Dammit. Orihime needs to marry someone and have babies with and keep the Ishida family going!!
And this kinda confirms my theory that these new guys are after Isshin to kill him for some type of "legal" reasons. But I'm like Ichigo right now. Where the hell is he? We need some more info here on Isshin! He's been mysterious WAY too long now and we need answers.