What is a showrunner? Is it just a word they invented for writer or director? Or is Showrunner the name of an old Heroes for Hire villain I'm clueless about? haha.
TV is vastly different from film. Directors are in charge of bringing together each episode, as if a film, and the showrunner is in charge of each season full of episodes. The Showrunner is a mix of a producer and a "meta-director" -- they shape the trajectory of the overall story.
If you're adding more Bat-characters, they should be unique. Zsasz doesn't have flare or personality enough to warrant getting a spot in a non-Batman-specific game. There are better gun-toting DCU characters than Hush for a game like this, too.
Clayface and Ivy are solid picks, for variety's sake. Their powersets would add to the game.
I dislike rewriting continuity, which they will inevitably do when creating this "new" MU. You can change the trajectory of characters without going New 52 or a larger scale One More Day on them. Retconning and erasing history is as lazy as it is insulting.
Only way I'd support this is if the characters all remember how things used to be, but just had the reality around them change.