Live at San Diego Comic-Con this weekend, NetherRealm Studios announced the release date for the upcoming Martian Manhunter character and John Stewart Green Lantern skin -- July 30th. There is no "Season 2 Pass" announced yet, so it looks like you'll have to buy the character piecemeal for now (or just wait for the Game of the Year/Komplete Edition).
Strangely enough, the DLC announcement just notes the PS3 and Xbox 360 platforms, so the Wii U version will probably drop at a later date.
Here's a half hour of the Arkham Origins Mr. Freeze DLC, if you want to watch that
11:45 AM on 04.15.2014