I don't usually post here, but this is an interesting thread. Personally, I don't like mixing fantasy and scifi. I also think that the whole nuclear holocaust post-apoc thing is getting old. So how about something slightly more relevant to our times...
50 years from now...
Global warming has led to the melting of the polar ice caps, inundation of coastal areas, runaway pandemics, and general economic hardship. In an attempt to stop further damage to the environment a group of ecoterrorists develop a genetically engineered bacteria that eats petroleum, reducing it to a useless sludge. Stressed world economies collapse and anarchy breaks out around the globe.
Flash forward another 50 years...
The player starts life as a farmer living in a small rural community under the "protection" of a warlord. His village is attacked by a mysterious raider (yes, I know it's stock CRPG plot at this point) who murders most of the men and enslaves the women and children. The player (be he male or female) is one of a few survivors who escapes, and is forced to seek the Warlord's aid in freeing the prisoners. The Warlord has different ideas, though, and "recruits" the player into his army...