It looks like I was right about the FGT, and the reason to why Isshin never used it against Aizen in FKT also the reason to why he might have lost his powers in the first place.
Unbelievable, the final Getsuga...
Ichigo: I'll show you, the final Getsuga Tenshou.
Aizen: What, that form is....
Underwater? Well, during that time in his mind--
Shirosaki: Haven't you fallen yet, Ichigo.
Ichigo: Don't make me laugh, I'm not down yet. Not until I get what the final Getsuga Tenshou is.
Shirosaki: I'm tellin' you... I'm not gonna teach you!!
Ichigo: (Why is it, that if he really wants to kill me, then he could've done so since the very beginning. He's got that much difference of power between us.
If he really doesn't want to teach me, then just stopping this fight and avoiding me would be fine. He could've done just that. So why
So why is it that from his sword, flows such overwhelming loneliness)
The two exchange blows.
Shirosaki faces Ichigo and separates him from his sword.
Ichigo is stabbed.
Shirosaki: So you've noticed... The final Getsuga Tenshou can't be learned from any blade but mine.
Ichigo: It doesn't hurt...
Shirosaki: Of course. This Tensa Zangetsu is originally yours... To be pierced by it will not cause any pain.
Tears spill from Shirosaki
Ichigo: Why the hell are you crying
Shirosaki: Ichigo, do you remember the words I said at the beginning of this fight? That what you want to protect is not what I want to protect.
Shirosaki: What I wanted to protect was... you, Ichigo.
Ichigo: What do you mean.
Shirosaki: I meant, that the main point of the technique is directly taught straight from this blade. Ichigo, to teach you this technique, you have to----
End of Flashback
Aizen standing in mid-air.
Ichigo: The final Getsuga Tenshou is to turn myself into Zangetsu. (His black hair flows all the way down to his hips, and bandages wrap around his face from the nose down like Kakashi)
Ichigo: This technique is, for me to part with all the power of a shinigami, that is what the final form meant.
Aizen: (Impossible. I who, evolved beyond the dimensions of a shinigami, able to an existence of two or three dimensions apart from it, could not cross such a line
to intentionally lower your own level as far as your limits could go, not even as a shinigami or a human, to the point where my reiatsu could no longer even register it
impossible. impossible. This bastard, this man, to stand and evolve beyond me is---)
Aizen: FOOL!!! That such a thing could happen!!! THAT SUCH A MERE HUMAN COULD SURPASS ME!!! THAT SUCH A THING IS EVEN---
Ichigo grabs his sword
MUGETSU!!!! (Moonless sky)
In the next page it opens with the sky filling with darkness
To bring everything back into darkness--!!!