I dunno if this has already been done, I didnt really search in-depth.
Though we all know that Isshin is quite possibly the most awesome character in Bleach, there are lots of things we dont know about him.
When Urahara was training Ichigo for the S.S. Saga, he said that he originally had shinigami powers, which had just been awakened by Rukia's donated ones.
That brings us to the conclusion that he's inheriting these powers from someone, and I dont think it's aunt Betty.
The suspicion that Isshin was more than just goat-face was increased in the more recent episodes and fairly old chapters, when he appeared, decked out in shinigami gear.
Now, not only do we know that Isshin passed on the power to be a shinigami to his son, but he also passed down a lot of it (Considering how easily he took down the low-class espada Aizen sent to earth).
Though we cant blame Isshin for all of Ichigo's insane amounts of reiatsu, we can blame him for some of it.
Not only his power displayed that he was powerful (..duh.) but so did the ripped Captains cloak draped over his shoulder.
Which raises the question, who is this man?
I'll post my findings in a little while, after I've heard a little bit of what you guys have to say.