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Hush is probably one of the most formidable opponents that Batman has faced. In the few appearances that he's had, he's been just as much of a physical match for him as he is an intellectual foe. Plus there's an arc of Batman comics that is widely celebrated that bears his name ("Batman: Hush"), which not only works as a comic title but could be perfect for the leap to the big screen.
Hush is the alternate persona created by Dr. Thomas Elliot. Elliot is a master surgeon who grew up in a wealthy home and was friends with Bruce Wayne at a young age. Having had to endure a life of comparisons to Gotham's favorite son, he began to resent Wayne and conspired with the Riddler to kill him. Setting out to destroy Batman, the pair use many different tactics to drive him mad and bring him to a breaking point, including setting up a situation so that it appeared Elliot was a victim of the Joker.
What is really interesting about this character is the thing that would make him very cool for a film, the fact that he comes from a similar background as Bruce Wayne but came up with different results. Flashbacks of the two as children and how Bruce Wayne indirectly influenced Elliot's downfall would perfectly set up his hatred and new persona. Plus with Hush on the table, it opens up the opportunity for something that we haven't seen much of in superhero films: villain cameos. The only ones that come to mind are Cillian Murphy in the "Dark Knight" trilogy, but this is something that happens in comics a lot, villains appear for a few pages or panels and only offer a little to the story. Why isn't this being exploited on film? Hush as a villain will not only bring entertainment to the film but also a chance to reference and introduce future villains.