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.
Well, start by planning out your story and deciding the key moments you want to show in each panel. Then, think about the flow and transition from one panel to the next to create a smooth movement.