I think it depends on his inspiration and motivation. If he has a story he's passionate about, there's a chance. But it could also be that he wants to take a break or focus on other things for now.
I don't know for sure. It depends on a lot of factors, like his inspiration and personal plans. We'll just have to wait and see if he makes an official statement.
I haven't heard any news about Hajime Isayama creating another manga. Usually, such announcements would come through official channels or reliable sources, and there haven't been any of those yet.
In the 'Alec gets manipulated by Magus' fanfic, Magus could use his charm to manipulate Alec. He might be very smooth - talking and convince Alec to do things that are not in Alec's best interest. Magus could also create situations where Alec feels indebted to him. For example, Magus could 'save' Alec from a made - up danger and then use that as leverage to get Alec to do his bidding. Additionally, Magus may manipulate the information Alec has access to. By controlling what Alec knows, he can control how Alec thinks and acts.
In "The Half-baked Feng Shui Master", a modern urban supernatural novel, the female protagonist was tricked by her demon-controlled boyfriend into another world, and the female protagonist became the ruler of the other world. The male protagonist of the novel is controlled by the devil, but the specific plot needs the reader to find out for themselves. I hope you like this fairy's recommendation. Muah ~π