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.
Yes, there might be. The Naruto and Attack on Titan fandoms are both huge, and fans often like to cross over their favorite characters in fanfiction. So it's quite possible to find Eren - from Attack on Titan - in Naruto fanfiction. However, it may not be as common as fanfictions within each respective universe.
I don't know what kind of cartoon the female lead was brainwashed in. Maybe it's a specific plot of a novel or a popular cartoon. If you can provide more information, I'll try my best to answer your questions.
Another possibility is that it could be an alternate version of one of Cross's friends. This alternate version has been corrupted in some way and wants to drag Cross down with them. They use their knowledge of Cross's weaknesses and insecurities to successfully brainwash him, believing that they are doing the right thing or that they are actually helping Cross in some warped way.