I think they should make the movie before the Justice League movie, using the Titans as a sort of precursor to JL, where after seeing these kids unite to fight crime, the high end superheroes resolve their conflicts with each other and are inspired to create the Justice League. After all, the chronology of the Titans has predated JL in some story lines, where the JL was not founded until Red Robin was Batman's sidekick, and Young Justice was not founded until he Titans were too old to be members.