I didn't think that The Green Lantern "flopped". It just wasn't a huge hit. After making its rounds on cable and DVD, I'm guessing a sequel would do far better, especially with a better script.
I finally watched "The Avengers" on blu ray last night and was extremely disappointed. Maybe I was in the wrong mood but IMO, it was inferior to all of the stand alone movies, save Captain America, which was difficult for me to get through (actually took a few nights).
A JLA movie might sound "bad" but I wouldn't and won't discount a Chris Nolan effort until he disappoints.