As the title says, while waiting for new chapters to come out I've started rereading the series, and I've come up with some theories about a few things. The first and most prominent one is about Ichigo's bankai. In Chapter 541 we learn that "Old Man Zangetsu" wasn't really Ichigo's zanpakuto, but rather the manifestation of his Quincy powers. The true Zangetsu was, in fact, Hollow Ichigo. The Old Man states that whenever he taught Ichigo about his zanpakuto, he had him fight against Zangetsu rather than himself. From what I've seen, this is true for all of the fights Ichigo had with his powers except for when he learned bankai (he also fought his Quincy powers when learning the Final Getsuga Tensho, but he fought Zangetsu at the same time). When he learned bankai, he fought against the Old Man, not against Zangetsu. While this could have just been a mistake on Kubo's part, or he's not counting it for some reason, it seems like he's had the series planned out entirely pretty much since the beginning. Which makes me think: what if Ichigo's bankai isn't really a bankai at all? We know that the Old Man has been holding back Ichigo's shinigami powers, so it makes sense that he would substitute what appeared to be a bankai with a Quincy power. It could potentially be a Quincy spirit weapon, like Yhwach's sword. Also, I've noticed that there are many instances where Ichigo breaks out his bankai without calling it out, something I don't recall seeing any other character do (granted that this could very well be a translation error or completely normal for bankai). I really hope that with Zangetsu's true form revealed and the Old Man not holding Ichigo back anymore that we'll see a new version of his bankai, regardless of whether or not the old one was a true bankai.
Next is about something a bit less exciting, but still pretty interesting: Aaroniero Arruruerie, the Ninth Espada. He is the only Gillian-Arrancar to be in the Espada during the Hueco Mundo arc. Gillians are formed when multiple Hollows feed on each other until they become one mindless, powerful being. I think that Aaroniero's two heads is a callback to this. Since Gillians don't have a distinct personality and are comprised of multiple Hollows, it makes sense for the only Gillian class Espada to have two heads to represent that. It's quite possible that Aaroniero did have a distinctive personality as a Gillian and was just never able to evolve into an Adjuchas, but it still seems to be a reference to having less of a singular identity that the other classes of Menos.
There are a couple of other things I know I thought of while reading, but I can't really remember them at the moment. So, if I remember any/think of any more I'll add them in later.
If it wasn't for the fact that we know Isshin can use Getsuga Tensho too, I would suggest that it was actually a Quincy technique, since it seems to be quite similar to a Quincy's arrow. It's also similar to a cero, which makes sense considering the fact that Ichigo's zanpakuto is part hollow, and we've already confirmed the fact that Ichigo's black Getsuga is extremely similar to a Cero Oscuras. I have three different theories on Ichigo's Getsuga Tensho.
1) Ichigo's shikai Getsuga is a Quincy ability. The Old Man was suppressing Ichigo's powers, and taught him Getsuga as his main technique. Ichigo's old shikai might not have even been a zanpakuto. We know that it didn't have a true basis as a zanpakuto, and that it was just him forcing his power into a form much weaker than it would be with a true zanpakuto, so it could have been a spirit weapon manifested by the Old Man, similar to my theory about Ichigo's bankai. Granted that there was the whole "red spirit ribbon" thing in the beginning of the series that kind of proved that it was his shinigami powers, but I'm open to the idea. Anyway, back to Getsuga. It's a Quincy ability, except for when Ichigo brings out his bankai. The first person to use Getsuga in bankai was Zangetsu, who is a hollowfied zanpakuto. Therefore, when he used it, he used it like a cero. When in shikai, it would be a Quincy technique, but in bankai, it's a cero. But then why would Isshin be able to use it? In this scenario, I think the best explanation would be that his soul was connected to Masaki's for so long that some of her Quincy powers flowed into him, giving him the ability to use Getsuga.
2) Getsuga is a cero-like technique in both shikai and bankai. In this case, the Old Man, again, was suppressing Ichigo's shinigami powers. However, since Hollows are like a poison to Quincies, he was unable to suppress Zangetsu's Hollow side. Therefore, Ichigo was still able to essentially use a cero in the form of Getsuga Tensho. This also explains why the Old Man was unable to prevent Zangetsu from forcing Ichigo to Hollowfy- it was like a poison to him. Why was Isshin able to use it in this scenario? Again, because his soul was connected to Masaki, whose soul was connected to White, and some of White's powers flowed into Isshin's soul.
3) This scenario is honestly most likely. Getsuga Tensho is a shinigami technique that both Ichigo and Isshin can use. No twists, no turns, no changing what we think we already know, everything's the same as it was. Though I still hope Ichigo gets some new powers. Since gaining his own shinigami powers in the beginning and discovering his original Getsuga, all he's gained is greater physical abilities and a greater form of Getsuga (other than when Zangetsu took over and started using a cero). Don't get me wrong, it's a great technique and it's still entertaining to watch Ichigo fight, it just gets kind of old when he goes back and forth between swordfighting and blasting Getsuga during a fight.
This next theory will be much quicker and less complicated, I promise. It's about Ichigo's new duel shikai. I think that the smaller sword, which represents Ichigo's Quincy powers (from what I've heard, anyway) isn't part of his zanpakuto. I think that it's a spirit weapon, similar to Yhwach's sword. When Zangetsu was being reforged with the addition of an asauchi, and Ichigo forced some of his spiritual energy into his new zanpakuto, he probably forced his own Quincy spirit weapon out on his own, along with helping to create his zanpakuto.