I was wondering that myself. A lot of stuff is Warner Bros. (or they've done a film with the characters already)- Suicide Squad, Iron Giant, Akira, Freddy Krueger (New Line), Lord of the Rings (New Line), Speed Racer, Mad Max, etc.
Not sure how this would work but some stuff is under Amblin (like Back to the Future). But there were a few video game characters here and there (I know Spielberg had tried to develop Halo a while back) and just from doing some quick Googling, Stephen King's Christine and the A-Team are at different studios as well. I wonder how much some of them cost and if there were any restrictions on what could be depicted?