You mean apart from the Wasteland series?
Why would Obsidian do a post nuclear holocaust RPG when there are so many other apocalypses, so many other Science Fiction settings, completely untouched? How about a revival of Dark Sun? How about something in the vein of Planet of the Apes or Waterworld, how about something on a different planet entirely?
Im thinking something a bit closer to home that would run along the lines of a more real world scenario on a larger continental scale for the mass multi players, instead of a one city platform built for a one man army against a glorified AI