What if Jack Nicholson wasn't cast as the Joker?
Somebody commented on AV Club that Jack Nicholson definitely shined a spotlight on the film and piqued general public interest in it in a way that maybe a less famous actor (like say John Lithgow or Tim Curry) probably wouldn’t have done. Nicholson would have been the difference between getting it on the cover of People Magazine before release instead of just in it, or leading off Entertainment Tonight instead of just a later segment.
https://film.avclub.com/1844175827
More importantly, getting a major star like Nicholson probably made the studio more open to letting a young wunderkind director like Tim Burton spend pallets of cash to make something truly new and original.