When the game first started, Arleen Sorkin had reprised her role as Harley Quinn. However, when creating new content, Arleen wasn't doing the voice anymore. Instead, they went for Jen Brown (Pyrrha Nikos of RWBY, Agent Carolina of Red vs. Blue)
What Could Have Been: There's a concept for a Sinestro Corps-based Iconic Battlesuit "Sinestro's Might◊. But with the first DLC (Fight for the Light) it is now the style set Sector Incendiary.
Also, the Flash in the game was originally intended to be Wally West (And likewise Hunter Zolomon as Zoom), but it was changed to Barry Allen (And again, likewise with Eobard Thawne as Professor Zoom, but in Zolomon's costume).
Concept Art was drawn for Vic Sage as The Question, but the one you encounter in-game is Reneé Montoya. Though Vic is mentioned in the Briefings found during "The Hunt" challenge mission.