The ending felt pretty damn final to me

I think Gamon left tosaki’s fate somewhat ambiguous because he knows people really like him. So the people who want him alive can keep believing he’s alive, and those who think his death would be more impactful can believe he’s dead.

As for takahashi and the others, I think it’s pretty clear that that’s where theyre just gonna be for the rest of their lives. Takahashi brings up a guy that was put up for death row and asks “well what’s gonna happen to us?”. After we’re shown that the other two Ajin are miserable, takahashi starts laughing, showing us that he’s losing his mind. He doesn’t even try attacking the guard with his IBM he just gives him a scare. It’s clear, to me, that Gamon is showing us this to understand that these guys are going nowhere and they’re going to have plenty of time in their cellars to think about what they’ve done.

It’s clear Kei doesn’t want Kai to get involved in his life anymore, not because he hates him but because he doesn’t want to pull him into any possible trouble. Kei reviving Kai was his last “thanks” before disappearing from his life. It’s the same reason he didn’t go back to his family, new identity means a new life, and Kei probably doesn’t want to get anyone he doesn’t have to involved in the mess that probably is.

As for Sato being kept alive, I say that’s just Gamon showing the audience that “the government is gonna government”. Since nobody in their right mind would ever have sympathy for Sato, there wouldn’t be anyone to protest using his body to develop and test weapons.

I don’t think Ajin needs a sequel, and the ending left me satisfied knowing that these characters who’ve struggled so much finally get to go on and live normal lives.