Concept art from Dead Nation developer Housemarque reveals the studio had pitched and possibly worked on Dead Nation 2, a game based on The Walking Dead, and a Superman-related project.
The images, posted by former Housemarque artist Amar Djouad, show that Dead Nation 2 would have moved far beyond mowing down waves of zombies, showing characters fighting dinosaurs and giant monsters in a post-apocalyptic city.
Based on the concept art, it's possible Housemarque's Walking Dead may have been a top-down survival-shooter similar to Dead Nation. (Of course, that's purely speculative at this point.)
Lastly, art from the Superman project, which isn't listed as a game on Djouad's website, depict an older, down-trodden version of the Man of Steel and nemesis Doomsday, implying a potential "Death of Superman" storyline.
According to a series of tweets from Superannuation, none of these games are currently in development, though it's possible that Dead Nation 2 may have been approved and then cancelled, rather than rejected.