Wade Wilson, aka the mercenary known as Deadpool, once had the chance to become Batman but completely ruined it, taking the Dark Knight's famous inspiration in another direction. Following his escape from the clandestine organization that gave him his powers, Wade Wilson was in search of a new identity and purpose when an omen came crashing through his window - one any fan of Batman: Year One instantly recognized.
Unlike other heroes, Deadpool's powers didn't manifest naturally or through a tragic accident, but due to prolonged experimentation at the hands of Department K. Though the villains were able to give Wade his characteristic healing factor, he was badly mistreated and tortured, ultimately leading a break-out from their facility. On the run and with his cancer now held at bay, Deadpool suddenly had the chance to chart a new course, but no clue what to do next.
In Deadpool #36 by Christopher Priest, Paco Diaz and Andy Smith, Wade flashes back to a time after he had escaped Department K, narrowly avoiding being brainwashed into becoming a government assassin. Free from cancer but badly scarred and having left his old life behind, Wade found himself talented, deformed, and lost, seeking any sign of a new purpose. One day during his television time, Wade gets such an omen when a bat crashes through his window in a recreation of the iconic scene in Frank Miller and David Mazzucchelli's Batman: Year One.
In Batman: Year One, an injured and dying Bruce Wayne seeks guidance after a disastrous first night out patrolling Gotham, from which he barely makes it home to Wayne Manor. Bruce seeks to honor the promise he made to his parents, but despite all his training, he's missing something crucial. Bleeding badly, Bruce begs his father for some sign of what to do next, and as if in answer, a bat crashes through his window. Bruce perceives this as a sign, finally calling for Alfred to come and offer medical attention so he can begin molding the Batman persona.
When the obvious homage happens to Deadpool, he can't help but say, "It's an omen -- must be!" in true dramatic fashion. But even Batman's origin story isn't enough to get Wade out of his slump, and he's later seen eating the bat, consuming it instead of taking it as a sign. When Wade later notices police killing an armed, fleeing suspect, he references Batman again by commenting that criminals are a superstitious and cowardly lot, but this still doesn't turn him into a hero.
Instead, this scene gives Deadpool the idea to become a freelance henchman, acting as the guns and muscle of whoever will hire him. It took years for Wade to actually become a true hero, going from an outright villain to an antiheroic mercenary to a semi-trusted ally of the Avengers and X-Men. Deadpool has made pop culture references part of his identity, so it's hilarious to consider that when given the opportunity to become Batman, Wade missed all the signs.