The Final Getsuga Tensho
One of the first things that came to mind when the whole Bachgetsu thing came to light was would Bachgetsu have to do with the FGT, why did they have to merge, and how would Isshin be able to do it, that is, if he actually did, his zanpakuto may have just been too uncooperative. I then rewatched the deicide episodes and came across this in the manga
Isshin specifically says your FGT. This means that either A. Every zanpakuto that can learn this, the "getsu's", has its own special version of the technique, or B. Isshin knew that his would be different because of his hollow and quincy powers, or maybe even both. Regardless, this seems to be a technique that has a massive drawback but unless Isshin lost his powers before, regained them, then lost them again, he didn't suffer the same drawbacks as Ichigo. However we've seen a technique in the series with a similar drawback.
Now, Ichigo being a quincy could possibly use this technique, except for the fact that Ichigo has no Sanrei glove. What the Sanrei glove does is essentially give the quincy more power by actually limiting it, the training they do allows them to obtain greater power. The quincy vollstanding is the evolution of letzt stil, which gives the user an ability called sklavlerai, something similar to the merging of Hichigo and Bachgetsu. Bachgetsu has limited Ichigo's power this whole time, and by letting it out, and then using Letzt Stil, he was able to reach a level of power unlike anything he had had before, but Ichigo had to accept his hollow/shinigami side first. It's important to note that when Ichigo used Mugetsu, he formed a sword out of reiatsu, rather than solid zanpakuto. I believe this to be the only time Ichigo used his quincy powers before recent events
When it actually comes to the whole losing powers thing this makes even more sense. When Uryu used the technique, he didn't lose his powers, he just couldn't access them, and Ryuken had to help by shooting him with a heilig pfleil, a quincy technique. Ichigo could never really access his shinigami power and only used what Bachgetsu allowed him to use. Ichigo had his power's restored via zanpakuto again, and again Bachgetsu was present to limit him. I think the conciousness of his quincy side is linked to his hollow and shinigami power, just as his parents are and as Ichigo has accepted him, he is able to assume dominance. Similar to Uryu being shot, the quincy reiatsu in Quilge's cage re-awoke his quincy powers.
This is just a theory and I'm fully aware there are holes but you never know with Kubo so give me your thoughts and ideas on it. Thanks for reading.