The following contains spoilers for Episode 6 of Bleach: Thousand-Year Blood War, "The Fire," now streaming on Hulu in the U.S. and Disney+ internationally.
Bleach: Thousand-Year Blood War is the final arc ushering in a new era of Bleach in the Fall 2022 season, but not everything has changed and not every fallen foe is gone. Plenty of familiar characters have returned, including fallen villains like Loly Aivirrne and Tier Harribel. Even Grimmjow Jaegerjaques is still out there, and might get a chance to meet their old boss again soon.
Ex-Captain Sosuke Aizen faced his final defeat when his hubris got the better of him and he united practically everyone against him. As of now, Aizen remains locked away in Muken, the ultra-secure prison under Squad 1's barracks, but that's unlikely to be how his story ends. Recent Bleach episodes have hinted that Aizen might return to play a part in this war, but it's anyone's guess what the famously deceptive man might do.
Why Aizen Might Become The Soul Society's Best Defender
In recent Bleach: TYBW episodes, the Quincy king, Yhwach, visited the underground Muken prison while his body double kept Captain-General Yamamoto busy in mortal combat. Yhwach recognized Aizen as a special war power and tried to recruit him to his side. The Quincy king argued that he and Aizen both seek the Soul Society's destruction; surely Aizen would want revenge for his defeat in the replica Karakura Town.
To Yhwach's disappointment, Aizen declined; he hoped they would spend little, if any, time walking the same path. Yhwach gave up and departed, but this ominous scene was obviously more than a fun cameo of Bleach's nastiest villain. Characters like Aizen don't reappear just to look cool -- something is bound to happen to him -- or because of him -- soon.
Despite his humiliating defeat to Ichigo and Kisuke, Aizen still has vaguely noble intentions. Fan theories hold that Aizen launched his scheme not to destroy the Soul Society, but to become the heavenly king and defend the three worlds from the Wandenreich's wrath. Aizen has been compared to Star Wars' Palpatine, who became Emperor so he could unite the galaxy and militarize it in anticipation of the Yuuzhan Vong invasion. Aizen is a monster, no doubt, but he is still a roundabout protector of the Soul Society.
Even if Aizen's own bid to become the new god of reality failed, it's only natural that he wouldn't support the threat he had intended to neutralize. On principle, that would be ridiculous. Given the chance, it's possible that Aizen might join his former Soul Reaper comrades and fight the Wandenreich.
Why Sosuke Aizen Is Still a Problem for the Soul Reapers
On one hand, Bleach's proud but desperate Soul Reapers need all the help they can get to fight off the Sternritter and defend the Seireitei. If pressed, they just might release Aizen and request his aid -- especially in light of Yamamoto's defeat at Yhwach's hands. Aizen is a liar and monster, but he genuinely wants to protect the Soul Society from the Sternritter, which may have been why he aimed to claim heaven's throne to begin with. But even if he can be trusted to fight alongside the Soul Reapers and help defeat the Wandenreich, he is still a formidable loose end.
Knowing Aizen, he will find a way to pressure the Soul Reapers into giving him something in return after the Wandenreich's defeat, should that happen at all. He could demand to have his old rank of Captain restored, or even make another bid for the heavenly throne. He may ask for a chance to fight and kill Kisuke Urahara, the kido expert who helped Ichigo defeat him many months ago. The Soul Reapers would likely turn down all these requests, leading to a standoff.
There is a possible compromise; Aizen had once unseated Barragan to become Hueco Mundo's king, complete with an army of loyal Arrancars. If Aizen is freed and there's no chance to put him back into Muken, perhaps he could be banished to Hueco Mundo, with that entire world being his new prison. Most of the strongest Arrancars are all dead, so there's no real threat of the Espadas 2.0.
Ideally, Aizen will be put back into Muken -- and the Soul Reapers will likely try it. Failing that, Hueco Mundo is the next best option, where Aizen is safely out of the way in a world the Soul Reapers don't bother with anyway. When it comes to Sosuke Aizen, nothing is ever easy. But the Soul Reapers have triumphed against impossible odds before and they can do so again, against enemies both old and new.