Even Post Malone’s character technically doesn’t die during the film’s runtime and bad guy Discoll is arrested and sent to jail, so the movie leaves a lot of people who could theoretically play a role or pop up for a cameo if a sequel were to happen and they had some germane info related to the case, of course.
Mark Wahlberg plays Spenser in Spenser Confidential, so if a sequel were to be greenlighted by Netflix he would definitely be the key ingredient to bring back.
However, the movie does a good job of giving major character moments to Hawk, Henry and even Cissy.
All of them are alive and kicking at the end of the movie.