If you're going to dream, dream big.
With that thought in mind, I made a new world map for Fallout 2. It's based on a satellite photo of the western United States (that I heavily reworked in Paint Shop Pro to add character and detail), and is the maximum size the game engine can handle (10 by 10 tiles). The full-size image would be 3500 by 3000 pixels, but since that's far too big to post here is a drastically resized overview (with an overlay showing the real-world areas encompassed by the new, bigger map):
I'm pretty happy with the level of detail I was able to achieve while reworking the map. For example, here's an actual size (as it would look in the game) slice of the map from the area around New Reno:
The graphics work for the map is all done; I sliced it into 100 tiles, made an updated vanilla FO2 art\intrface.lst, and made all the .msk files. What I haven't done, however, is begin the arduous task of making a new worldmap.txt file both to integrate the new map into the game (existing town placements will need to be changed slightly) and provide sensible random encounter content for all the vast new swaths of wasteland on this map.
Eight years later, a download of the materials for this project is finally available here . The zip includes the map and mask images as well as the game-ready files in frm and msk format. Some brief instructions are also included on how to get them into the game. Enjoy!