User blog comment:Rassilon of Old/Young Justice spoilers at Comic-Con 2012/@comment-5339542-20120730161617/@comment-1038387-20120730165943

Ent warning.


 * Firestorm (Ronnie and Stein) appeared prominently in the later series of Super Friends. He was off-limits for Justice League Unlimited. (Ronnie and Jason) appeared in Batman: The Brave and the Bold. That's two out of three of the major DC Universe shows. (Not counting YJ yet)
 * In both cases, the merge partner was portrayed as a translucent floating head.
 * I'm not sure on the rights, but he is likely available.
 * I kinda disagree on the reasoning "Jason and Ronnie are both in teen age therefore better suited". Point of the character, originally, was that Stein helped Ronnie out whenever he didn't know how to do things. Teacher/student. That's probably why Jason went through so many merge partners.
 * A floating head is also a great exposition device.