Batman's Ending
Blackest Night
Crisis on Infinite Earth's
Flashpoint
Superman's Ending
Other
I think Mera is probably the most likely bet after the success of Aquaman and because of how popular she is. Ocean Master might be a skin for Aquaman.
Wouldn't be surprised if we finally get Beast Boy in there to close out the Titans in the Injustice universe.
Also for synergy with the TV shows and movies I could see them bringing back Shazam and introducing characters from the Doom Patrol as well as Stargirl. Maybe Huntress for BoP?
Actually addressing the elephant in the room regarding where the heroes are now.
The most disappointing thing to me about Injustice 2 was that it treated its world like an afterthought in favor of Braniac. Batman is now top cop and Superman was a dictator. With Batman developing Brother Eye and having his own prison for the Regime members (which includes his own son), how does the world get along with that? The world of Injustice had a point; how many times is Batman going to let the Joker live to murder, maim, or nuke another city? The problem was that Superman went way too far. What is the balance here if there is any and what does the lay-person feel about Batman or Superman's regime?
Just brushing it aside for Bruce to just be doing better is dodging the question and the more interesting narrative than pulling them together and then having to remind ourselves that they hate each other at the end.
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For anyone that needs to know why OMD is awful please search the internet for Linkara' s video's specifically his One more day review or his One more day Analysis.
"It's too bad she won't live! But then again, who does? - Gaff Blade Runner
"In a short time, this will be a long time ago." - Werner Slow West
"One of the biggest problems in the industry is apathy right now." - Dan Didio Co-Publisher of I Wonder Why That Is Comics
The good guys searching the multiverse for a good Superman to replace theirs.
Assassinate Putin!
Injustice 3: Heroes In Crisis :P