Imagine the sheer size of the playable map if it is full-scale to real-life.
It would either be unrealistically small or you would
have to use cars and then some (or I guess the world map could still be 2D overview but then the encounter maps are 3D?).
Sounds interesting, though even if some group did manage to recreate the world map with random encounters/good content, I have the feeling the some of the towns would just look weird in 3D. For example, the playable New Reno is quite small, and NCR was said to hold thousands of citizens. Not to mention San Francisco or making "city ruin" places outside of cities.
But ignoring the cities themselves, having a full-scale Fallout/Fallout2 map with realistic terrain (mountains, cliffs, etc.) that you would have to drive around and explore would be amazing. Caves, too, even if there realistically aren't a whole lot of those.