A sequel script is a written story or screenplay that continues the events of a previously made movie or book. It's the blueprint for creating the next part of a series or franchise. Full definition
«Zombieland 2» will most likely get underway once Reece and Wernick finish their other sequel script, «Deadpool 2.» (cbr.com)
This was also the subject of a potential sequel script written in the mid-90s, which was likely from from Robert Rodriguez (Desperado, Once Upon A Time In Mexico, Sin City, Planet Terror). (omegaunderground.com)
There was talk of a power play on the part of novel author E.L. James, who reportedly wanted to write the sequel script herself, the better to keep it close to the novel. (slashfilm.com)