With great reviews on the animated movie Superman/Batman: Public Enemies. It seems like DC animation is considering their first sequel to their direct-to-home animated movie slate.
The planned sequel for Superman/Batman most likely will take from Jeph Loeb's series, or other writers from the subsequent stories. Though it is also possible to get from older team-ups considering these two have been doing this for about 7 decades already.
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So the problem is not lack of stories but how to choose one. I tip my hat to DC in adapting some of their famous storyline into these animated movies, it helps introduce in a new generation of FANBOYS!
-access out, God speed
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