It's still stunning to think that Warner Bros looked at Batman Returns and saw a failure of a movie. Sure, it didn't make anything like The Dark Knight money at the box office and critics weren't ALL entirely enthusiastic, but it's pretty roundly now considered a masterpiece and it was critically acclaimed for the most part even at the time.
The problem with it was something else entirely. It was just too damn dark.
In a film marketplace looking towards merchandising dollars and the valuable family demographic, Tim Burton's dark circus of freaks was an oddity that alienated the very people Warner Bros wanted to engage most. And there was nothing left for the studio to do than move the visionary director sideways into a production role (though he had very little real influence on the Batman sequel that followed) and seek a lighter tone with Joel Schumacher.
The result was Batman Forever, a more conventional superhero blockbuster that made Warner Bros more money (certainly in merch terms) and which was best described by Roger Ebert as "bubblegum for the eyes." Its critical mauling suggests it was far from a success, though, and you really have to wonder how Burton's initial vision for a third Batman movie would have been received.
Luckily, we have some clues as to what it would have looked like...