I think Steppenwolf is a likely DLC choice to promote the Justice League movie in November, like how Zod came out near the Man of Steel release in 2013. He could be interesting with his electrified axe and Parademon army.
Also, October 2017 is the 25th anniversary of Mortal Kombat, so that's as good a time as any to release the inevitable MK guest character.
DC are so weird with their movie promo stuff. The first two Arkham games had nothing from the Nolan movies. Arkham Knight got BvS stuff in there ahead of release at least, and a TDK skin finally. Zod in Injustice came a month after Man of Steel, I guess developing him took longer than expected because that's a weird time to drop him. And I think the MoS skin came late?
This debate about the Season Pass just makes me dislike the idea of the incoming Guest Characters even more. There are so many interesting characters within the DC Universe, without having to draw on other external material.
Just resign yourself to the idea that there's going to be three guest characters and be happily surprised if there's less.