Guardians Of The Galaxy Vol. 3: Everything We Know
7. The Script Has Already Been Completed
Months before he was fired, Gunn turned in a completed draft of his Guardians 3 script, something confirmed by Marvel Studios president Kevin Feige while on the Ant-Man and the Wasp promo tour (via Collider):
“[Writer/director] James [Gunn] has delivered a draft and we begin official pre-production on that very, very soon."
Even when Gunn was fired, the plan was still to use his script. This would have ensured that - even with a different director - the Guardians trilogy still felt like a cohesive whole. But with Gunn back in the director's chair, consistency between the three films is a concern no more.
Since he's already finished the script, it's now just a case of polishing and tightening the story he already has, working towards that final draft. In a weird turn of events, his firing might have actually helped make the script better, since he's had several extra months to think of ways to improve it.