The first film, written and directed by David Ayer, earned US$747 million in 2016 but drew withering reviews. It is possible that it could be a continuation of the franchise with a new cast of incarcerated supervillains. But it also looked like O'Connor was moving on to reteam with Ben Affleck on the sports drama The Has-Been. Gunn's Suicide Squad could conceivably differ from Ayer's in terms of story alone, thereby employing numerous first film's same cast members and characters.
According to Variety film reporter Justin Kroll, there's another reason why O'Connor is no longer behind the helm: his story for Suicide Squad 2 apparently "ended up being nearly identical to the Birds of Prey story". What Gunn did with Guardians had a ripple effect on the whole of the Marvel Cinematic Universe, influencing how filmmakers approached their projects in the years following Vol. 1's release.
The director was sacked from the Disney franchise of films after tweets from years ago saw revealed jokes in poor taste, concerning rape and pedophilia - since then, Gunn has apologised and said he had grown as a person in the years since the jokes.
Suicide Squad 2 has suffered many setbacks since it was initially announced. Nothing but originality from this guy. Maybe, Gunn can improve the series in his sequel.
The film adaptation of Suicide Squad was already a bit of a Guardians of the Galaxy knock-off: you have a ragtag team full of scoundrels, banding together as unlikely friends to defeat a more powerful evil, all while classic songs play in the background. As of right now, it sounds like they'll be using his script, but they have yet to find a director.