I'm not actively looking to take away the sense of freedom given to players, so much as guide new players. 

Total freedom for an incoming player was lost the moment the first character sheet was accepted, and even further, the minute the IC was established and plots were set in motion. So, unless we're opening up to multiverse shenanigans, they'll still have to potentially contend with the fact that, in addition to the big three being taken, most of the concepts don't allow for their legacy characters.

For myself, that was stated right in the original CS that wouldn't be okay with someone applying as a Supergirl or Superboy, so that was a freedom taken right off the table from the get-go since a lot of my concept is hinged on Clark being the only Kryptonian in the galaxy. Likewise, I'm sure with Thomas Wayne as Batman, and an active Dick Grayson Robin, there's not exactly room for Tim, Jason, Damian etc in the game. 

So coming in blind and then being hit with disappointment isn't productive for a new player either. It's not that I want to railroad anyone into 'you need to apply for Green Arrow' so much as, 'hey these are characters we don't have that we think could be a great fit, but if there's someone else who isn't taken, we'd be happy to entertain that too.'