Huh, so this Kickstarter looks interesting: https://www.kickstarter.com/projects/mikegnade/maximum-apocalypse
I dig the art, and I like the idea of the apocalypse being the result of a different aggressor. I fear that expansions may be tricky because they covered the three big ones. I suppose the mutants could have a new face with a nuclear winter. Global warming could be another cause, though I can't think of many monsters to go with that.