It has been confirmed through a leaked interview of Shintaro Kawakubo, who is the editor of manga Attack on Titan written and illustrated by Hajime Isayama, that author have no plans to write a sequel to Attack on Titan series.
In interview, editor Shintaro Kawakubo also revealed some important information about Hajime Isayama future works, Here is the short summary of the interview:
Did the ending of Attack on Titan play out as planned from the beginning?
This is how we planned the play to end from the beginning. The events in between were established during serialization, but how the main character would end the story had already been decided from the beginning, and that never changed. To be honest, Isayama sensei wrote the story always with that in mind.
Did the content of the story itself change at any time?
The content itself never changed, and although Isayama felt a great responsibility for it, he never thought of changing that ending.
Did you think it would be a revolutionary work when it started?
He knew it was going to be a masterpiece when he was with him, and he wrote in the headlines of the second or third volume that this was the path to the true shonen manga of the 21st century, but being an ancient story, a story of isolated people who they fight for their freedom. However, strictly speaking as an editor, I didn’t think it would be “revolutionary.”
What was the part of the manga where Isayama had the most doubts?
The part where Sasha saves the girl. In the drafts it was a story where Sasha did, in fact, die. But what was the point of her dying at that point? I asked Isayama. He seemed concerned about it, so in the end Sasha survived there.
Is there any chance that a Shingeki no Kyojin sequel or spinoff will be written?
Not actually. There is no reason there is any sequel or derivative work. We have heard many requests in this regard from fans, but we regret to tell you that it is not possible. In fact, Isayama is pretty exhausted right now, so even his next serialization is undecided yet.