Warning: contains spoilers for Bleach
As a self-professed immortal, Bleach's main antagonist Sosuke Aizen has always posed a significant threat to Soul Society even when he's imprisoned, especially as he's proven himself more than capable of breaking out before in the original manga. Fans therefore always knew that he could escape again should mangaka Tite Kubo ever decide to relaunch his popular series. That may have very well just happened, for in his one-shot manga set after the events of Bleach, Kubo sets the stage for Aizen's return, but not in the way that fans may have originally suspected. In fact, Kubo puts Aizen's immortality into question.
Aizen doesn't actually appear in the newly released Bleach one-shot. Instead, the fallen Arrancar Szayelaporro Granz hints at the former Hollow leader's supposed return after emerging from hell to confront a group of Shinigami including Ichigo and Renji. The main reason for the Arrancar's arrival, however, isn't just to impart the terrifying news about Aizen but to reveal the unfortunate truth about a Soul Society ritual known as Konso Reisai, the performance of which has made the Shinigami the unwitting participants in the damning of their fallen comrades into hell. In typical theatrical fashion, Szayelaporro finishes one such ceremony that the Shinigami had started unbeknownst to them to cast the soul of the late Captain Jushiro Ukitake of the 13th Division into the fiery abyss.
To further terrify his captivated audience, Szayelaporro divulges even more horrendous news, during which time he mentions Aizen and another prominent villain by name: "The powerful spiritual pressures that were holding the maw of hell shut....Sosuke Aizen and ..... Yhwach, have disappeared." Thanks to his fragmented speech pattern, Szayelaporro's words are quite ambiguous. He could very well mean that either Aizen and Yhwach were the spiritual pressures that had been holding the maw of hell shut until recently or that the spiritual pressures that were holding Aizen and Yhwach in hell are now no more. The former is unlikely because it doesn't make sense for Aizen and Yhwach to keep hell shut. But the latter would be utterly baffling if it were true because it assumes that Aizen was in hell, meaning that he must have died. But isn't he immortal? Conversely, it's believable that Yhwach would have been in hell. It was understood that he perished at the end of Bleach when Aizen senses the disappearance of Yhwach's Reiatsu 10 years later. Either way, Aizen and Yhwach are two of Bleach's most powerful villains, meaning nothing good can come of their release.
Regardless, if this one-shot manga does in fact launch a whole new Bleach series, fans can possibly expect Aizen and Yhwach to make an appearance. If they do, they will either have had to of found a way to set aside their differences about combining all three planes of existence to fight alongside each other against Ichigo and the Soul Society or still be so opposed to each others' conflicting philosophies, forcing Aizen to fight alongside Ichigo again. However, the latter is less likely as that would just be a rehash of the Thousand-Year Blood War arc. In fact, it's possible that Yhwach might have a different opinion of hell and no longer wish to eliminate the fear of death, seeing as Szayelaporro had some fond comments to say of his time there.
Even if Aizen and Yhwach do team up, hopefully, Sosuke Aizen's mortality will actually be confirmed because he has been overused extensively and just needs to disappear for good. It still feels this way, even after the extended manga hiatus. But fans will undoubtedly accept anything so long as Bleach continues.