Well, that was interesting.
I find it interesting that this Hollow has had its hole healed/sealed. Before this, every Hollow, even Arrancar, were set apart from other races by the gaping hole somewhere on their body. The fact that this Hollow is uniquely whole intrigues me greatly, since this is set in this past and the Arrancar later created by Aizen were missing this feature. I guess we'll see what that means, since it was important enough for Isshin to point it out. Also, the Cero it fired was apparently different in some way, and it fights more like a Shinigami than a Hollow - the exact opposite of Ichigo with his Hollow mask on, which is interesting.
Speaking of Isshin, Kubo was nice to us and gave us his release command and Shikai appearance. Now, though the command is "Burn", I'm not entirely certain if it's related to fire, and hear me out on this. Isshin can use Getsuga Tenshō, and I can't recall a Zanpakutō which possesses both a regular old melee ability AND a Kidō-type ability. Though there was stuff hovering around the blade, I'm willing to bet it's Reiatsu, which indicates he's about to use GT. I guess we'll see, though.
So Aizen and the rest of Underdeveloped Antagonists Inc. have showed up, with Aizen cutting Isshin's cheek to show him he means business. Because we all know the cheek is the vital organ of a Shinigami. But yeah, Isshin knew not only of Aizen's scheming, but of Gin and Tōsen being part of it. I guess to most people that would've been obvious, but it's nice to have it confirmed. It looks like Aizen wants to get up close and study the fight. Why he wants this is a mystery to me, but I wouldn't be surprised if it provided data for Arrancar creation and fighting capability. Also, apparently Urahara's Reiatsu-blocking cloak is public domain. Urahara needs to learn how to patent this stuff.
We got a little dialogue between Masaki and Ryūken, and apparently, there are two types of Quincy: pure-blooded and half-blooded (according to the scan I read). I assume the purebloods are more powerful than the halfbloods, but it could mean something else entirely. We saw Masaki's personality at play, which is nice, since all we knew is she smiled a lot and sacrificed herself and whatnot.
Overall, not too bad of a chapter. It seems Kubo has set a good pace for this sub-arc, and hopefully it'll continue at this rate. Feel free to leave comments on this stuff, as always.