The Guardians filmmaker is currently gearing up for Peacemaker Season 2, which will help expand the DCU. The first movie Superman featured a number of other heroes, but it's unclear what the plan is for Aquaman. While on the Peacemaker podcast, Gunn was asked about about the character formerly played by Jason Momoa, and he said:

The mural ends with a painting of Sean Gunn’s character Maxwell Lord, whom we know is the founder and financial backer of the Justice Gang in the DCU. Lord eventually performs a major heel turn in the comics, but we’ll see if the same holds true for this cinematic universe.

Some of the initial guesses for the heroes in the Hall of Justice metahuman mural in Superman were spot on, while others were way off.