Co-op would definitely be cool, unfortunately I don't see myself having the time (or dev skills, really) to get it done added anytime soon. The normal (single player) game still has a lot of work to be done - so I'm focusing on that :)
Hazardous weather (sand storms, rain/thunder, meteor showers) are on my list of ideas to try out.
"Aliens" haha mmmaybe in the form of some sort of passive, occasionally aggressive, creatures. As long as I can keep it from becoming a top-down shooter kinda game. Haven't really given it much thought.