Michael Keaton had wanted his third outing as the caped crusader to be an origin story for the Batman. He had envisioned something like Christopher Nolan's Batman Begins.
Before there was Nolan & Bale, there was Burton & Keaton. In 1989, director Tim Burton and actor Micheal Keaton had made the very successful Batman, which featured legendary actor Jack Nicholson as the Joker. It helped to make the super hero movie genre credible. The popularity of the film led to the 1992 sequel Batman Returns. After that, Burton left the franchise. Michael Keaton had his own ideas for what the third film should be about. He envisioned a prequel that told the origin of the Batman, similar to Christopher Nolan's Batman Begins. Director Joel Schumacher took over the franchise after Burton's departure and had very different ideas to what Keaton wanted to do, which was part of the reason Keaton left the series.
On a recent podcast interview, Keaton said, "It’s easy enough to look at what Nolan did and point to it thinking 'Yeah, this is what I wanted to do a decade ago'. You look at where he went, which is exactly what I wanted to do when I was having meetings about the third one. I said 'You want to see how this guy started. We’ve got a chance here to fix whatever we kind of maybe went off. This could be brilliant'. Joel Schumacher didn’t want to do it, so I didn’t want to stay."
Keaton was originally lured to the Batman film series by being shown the critically acclaimed graphic novel "The Dark Knight Returns" by Frank Miller. The Burton films, like the Nolan films, were in the spirit of Miller's popular story, but Schumacher preferred a more campy approach to the material. The franchise went downhill from there. However, its probably safe to say that we never would have had the hit Nolan Batman trilogy if it hadn't been for the success of the Burton/Keaton Batman in 1989.