Ra’s Al Ghul
If Warner Bros. really wants to make a movie about a villain with a mysterious past, a far better option than the Joker is the infamous Ra’s Al Ghul. Leader of the League of Assassins (or League of Shadows in some variations), Ra’s Al Ghul is a brutal villain and an incredibly formidable foe who has been around for hundreds and hundreds of years. He usually has two goals in mind: wipe out potential threats to humanity (usually meaning the scum of Gotham City), and finding someone worthy to take over as Ra’s Al Ghul.
This is an interesting subtext of his character that isn’t explored often. Although Ra’s Al Ghul would never just give up his title to someone unworthy, that doesn’t mean he wants to do this forever. A prequel movie about The Demon’s Head (one of his many nicknames) could follow a young Ra’s Al Ghul stumbling upon the mystical Lazarus Pits that grant him immortality and setting out to do some good with his newfound gifts. But over time, the Pits would corrupt him, turning him into the villain we know today.
Because Warner Bros. is so adamant about confusing audiences with continuity, they should just go for broke and get Liam Neeson to reprise the role he played in Christopher Nolan’s Batman trilogy. Because, why not?