An image of the back cover for Batman: Under the Red Hood reveals the name of the next DC Universe Animated Original Movie:
The first Superman/Batman movie adapted the first storyarc from the popular Superman/Batman comic book series. It even used artwork of Ed McGuinness, who illustrated the story, as the style for their characters.
The second storyarc in the series is The Supergirl from Krypton in which it is revealed that the meteor seen in Public Enemies was carrying Superman's cousin, Kara Zor-El. This is the modern age (post-Zero Hour) origin story for Supergirl.
Wonder Woman is a major player in the story as well as Darkseid, whose home dimension is Apokalips. The Superman/Batman: Apocalypse title makes me think that this will be the featured story, but this has not been confirmed yet. I'm sure we'll find out soon enough.
The art for this storyarc is by the late Michael Turner. While I think that an animated adaptation of his art would be amazing, I kind of hope that the designs used in the last film carry over to this one. I'm used to artists changing for every storyarc in the comics, but I think I'd like a bit more continuity in this film series.
But whatever happens, I'm sure it will be amazing. These DC animated movies haven't let me down yet. Superman/Batman: Apocalypse is scheduled to arrive Fall 2010.