8. Prometheus And Black Mask - Batman: Arkham Knight
If there's one thing Batman: Arkham Knight was lacking, it wasn't content. The game was filled to the brim with all sorts of side-missions, easter eggs and gameplay features, which was further expanded upon in a well-stocked (albeit controversial) season pass that released a few months later.
While the Matter of Family add-on definitely took the cake for the best of Knight's post-release content, the Season of Infamy that followed it was a close second. The game's final add-on bolstered its roster of villains with Killer Croc, Mad Hatter, Ra's al Ghul and Mister Freeze all joining, but according to in-game files, there were two other villains who were originally meant to join them.
Prometheus and Black Mask were both apparently meant to be included in the Season of Infamy, but were cut during development on the project. That's a massive shame, given Prometheus himself was a long-requested character for the series after nabbing an easter egg all the way back in Arkham Asylum, and also after Black Mask only got a minor role in Arkham Origins.
It is possible, however, that Black Mask's appearance was re-tooled into the Red Hood Story Pack, which ends with his death at the hands of the eponymous vigilante.