Chris O'Donnell's performance as Batman's perennial sidekick-cum-trapezist has been roundly criticized and mocked ever since the release of Batman & Robin, (even if the film proper has gotten a minor reappraisal in recent years) so much so that the Boy Wonder hasn't appeared in live-action (on the big screen at least) since O'Donnell put down the cape. However, had things gone according to their original plan, comic book movie history might've played out very differently.
Actor-comedian Marlon Wayans, probably best known for his starring roles in Requiem for a Dream & Scary Movie (and that film's numerous clones and spin-offs), was originally cast in the role during the pre-production of Tim Burton's Batman Returns, released in 1992. As Wayans tells it on a February 2018 episode of The Tonight Show,
What happened was, I auditioned for Tim Burton, I got the role, but it was the second Batman [Batman Returns]. There was Catwoman, there was the Penguin, there was all these different characters they were introducing.
Producers got cold feet due to the presence of so many new characters to the still-new cinematic Batman canon, and so Wayans' part was excised in on-set rewrites.
...they felt it wasn't enough time to put Robin in the movie. So they wanted to do Batman & Robin as the next movie.
Wayans was still paid his $100,000 salary for the role, but ultimately when it came time to bring Robin to the big screen three years later in Batman Forever, new director Joel Schumacher opted to recast the role with Chris O'Donnell, then a promising up-and-comer with a Golden Globe-nominated turn in Scent of a Woman.
Wayan's Robin would've been introduced as a garage mechanic with a familiar-looking 'R' stitched into his overalls, who helps Batman remove the Penguin's (Danny DeVito) remote control device used to hijack the Batmobile near the film's halfway point. This Robin would've featured prominently in the climax, infiltrating the Penguin's layer via a special underwater suit, as seen in this early concept sketch courtesy of artist Bart Sears.
This Robin would've been a darker, more brooding character that the one seen in Batman Forever, chastising the Caped Crusader for not killing criminals.
This wasn't the first or final instance of mid-production re-casting or re-writes. Tim Burton famously set aside the original script by Batman '89 scribe Sam Hamm (which featured Harvey Dent in the Max Schreck role, an outright murderous Catwoman, and buried treasure in the Batcave) for one by Heathers screenwriter Daniel Waters, and later hired an uncredited Wesley Strick (of Cape Fear fame) to rewrite much of the third act.
On top of that, Burgess Meredith, who played the Penguin in the 1966 Batman TV series, was cast as the new Penguin's father, but was replaced by Burton regular Paul Reubens after falling ill. Sean Young, who had been cast as Vicki Vale in the first film but had dropped out due to a leg injury in pre-production, infamously showed up to Burton's office in a homemade Catwoman costume demanding an audition.
But it was Wayans' lost role as Robin that would have been the most transformative. And as such, it's the biggest missed opportunity of all.