Here's a thought: no laser weapons. You know how in Halo games it is the 24th century, and the earthlings never mastered laser weapon technology, so they still used mainly bullets and gunpowder? So here's a thought: we could have something similar to that and not have lasers but rather bullet guns. It seems more realistic this way, because the people on earth would never have time to build on laser tech because of the whole "end of the world" thing.
-Rhino