Just some random ideas for a post apocaliptic world, either through war or a zombie outbreak or other forms.
1/ Geniticly recreated dinosaurs break out of a lab and begin mass breeding around the world, causing people to take shelter in whatever building they can. This rp wpuld take place either in a city or in the research facility. Characters would most likely be special agents trying to look for survivors or survivors who are trying to stay alive and figure out how to get to a safe place.
2/ Zombie infestation- a little like the movie zombieland where a disiese like rabies turns people into zombies and either a small group or two people are moving across the country, or just trying to survive in one city till the army irradicates the infestation with a cure.
3/ After a nuclear war people have begun to resurface from underground shelters after about a century passes and try to re-adjust to surface living, however there is still (in some areas) the threat of mutation from the radiation creatingmonsters out of any creature unfortunate enough to go near it.
5/ After a litteral appocolipse with the four horsemen the survivors left behind have to start from scratch, re living the middle ages but in the remains of the modern world. All modern machenery has been destroyed so old fashioned weapons are in use.
If you like one of these ideas or have one of your own pm me, I have one or two characters I'd like to use but if you would like to request something feel free.