After watching 302, I realized a very important thing. I know I may sound I'm just speculating but to put it in a more positive way, I'm just connecting the details.
First, Isshin mentioned that Zangetsu and Engetsu won't teach Ichigo and him the FGT because of some reason. During Ichigo's transformation, TZ said that the thing he wanted to protect was Ichigo. And since Ichigo will lose his shinigami powers once he enters the FGT mode, TZ will lose Ichigo. That's why he doesn't want to teach it to Ichigo.
Second. Since Isshin knew about this, I think it's safe to say that he either got the information from Engetsu (which I doubt) or he personally experienced it or used it. Now, if I'll go with the latter, I think that explains why Isshin lost his Shinigami powers. Although the issue of him being ceasing as a shinigami and losing his powers (as stated in his history section) might contradict my statement.
Third. Here we are with the new arc. Kugo shows Ichigo Isshin's photo and tells Ichigo that there are things in his family that he doesn't know. Then, the latest statement Kugo had was that their (Xcution) purpose is to restore Ichigo's powers. Now, WHAT IF they restored Isshin's powers? Or maybe they didn't because Kisuke mentioned Isshin "abandoned" his shinigami powers 20 years ago (arrancar arc section of isshin).
These are just my thoughts. I don't mean to post "stupid" blogs. I am only expressing. Any thoughts?