I want to make a long, cinematic zombie movie. I already kind of thought of a script, but I haven't thought of any details.

story so far: they invent a new spaceship and they go to some distant planet. When there, the (lone) astronaut gets attacked by some aliens and barely escapes. When he gets back to Earth, he is hospitalized and his condition deteriorates. Then he turns into a zombie and the virus spreads. eventually, after alot of stuff happens, the world population is reduced to mere millions and the world enters a new dark age where small clans of people compete for power while holding off zombies. Then some more stuff happens and the zombies get killed and civilization restarts. 

Can you guys suggest some ideas to fill-in the story-line? thanks. (P.S I'm pretty new to the forums here)