I think they may have waited too long to make this movie. I mean The Simpsons have been on the air for like 15 years now. The current series seems to be lacking in creativity lately. If they can't make a half hour show work, I'm not seeing how they can make a full length feature work. Maybe this is a project that should have been done years earlier when The Simpsons still were still fresh with ideas.
I'm sure the fans will come just because they are fans so I don't think the movie will tank at the box office, but I think it won't be as great as it could have been if it were produced years earlier.