6) Robin/Nightwing
There are two ways that Robin could turn up in Justice League. The first option is we see the flashback to Jason Todd’s death at the hands of the Joker. This iconic event from the comics was already confirmed to have taken place in the DCEU as Todd’s Jokerized outfit is shown in the Batcave in BvS. Obviously, the most natural home to explore this backstory would be in The Batman, but there’s nothing stopping Snyder from dropping another hint at it in this movie.
The second option is Nightwing turns up. The existence of Jason Todd in the DCEU also suggested that his predecessor Dick Grayson was running around somewhere. That was confirmed when it was announced that a Nightwing movie was in development. Seeing as that movie wasn’t revealed until after Justice League had finished filming, and that no casting announcements have been made so far, it’s unlikely we’ll get to properly meet Grayson in the film. However, there’s nothing stopping him from turning up in a flashback to his time as Robin or perhaps even just in silhouette form.