User blog comment:TVthePunisher/Final Getsuga Tensho and Isshin/@comment-1268838-20100916124921

I don't know, I just can't imagine Bleach without Aizen or the Hougyoku. Up to this point and a century before the story even started, those two have been the axis around which everything bad that has happened. Any event in the story was simply a matter of "The Hougyoku did it" or "Aizen planned it." Throwing those two out the window means that whatever new baddy pops out is going to have to start from scratch, unless some vague mention of him/her/shim/it was made at some point in the series. The only way I can see Kubo handle the story without plot suicide is to either preserve Aizen or the Hougyoku - the latter seems to be more likely in my book. Whipping a new villain out stronger than either of them would just make the series stale, and make everything the protagonists have achieved seem worthless.

More on topic, I agree that Isshin most likely lost his powers by using the FGT himself; having lost his powers and have the ability to teach Ichigo the method of obtaining it is too much of a coincidence. As for WHAT he used it on, it's hard to say... but I really hope Kubo doesn't use such a vague plot device to justify a new enemy.