This sounds really interesting, but I agree with the suggestion of creating a new story of it instead of using the already developed stuffs. And if it's set in just one part, it'd end up getting to the point where we wouldn't be able to continue. Perhaps it could even be set in the same world, but with a whole different story. Like, perhaps it could be set 20-30 years prior of the actual show/manga, but with an alternate timeline where the walls were invaded then. If we did that, we could create a whole new story without worrying about inconsistencies and getting to a point where we couldn't continue.