The University of Tokyo Newspaper has run an interview with one of their economics majors who went on to edit a certain manga that was the #1 seller for 2014. In addition to offering general insights about the business and role of an editor, Attack on Titan's Shintaro Kawakubo gave his perspective on plans for the series and on author Hajime Isayama.
According to Kawakubo, the manga is finishing laying out its mysteries. He says he think it will take three or four years to address them all and wrap up the series.
In terms of process, he says that Isayama works on the storyboard at Kodansha, and because the series is a monthly, the editor and author meet to discuss them over the course of seven to ten days. He reiterates that for all the series' success, Isayama has remained very humble, for example expressing thanks for small things.
Kawakubo mentions that his favorite scene of far is the one where Annie reveals her big mystery to her 104 comrades.