I personally dont think it would be a good idea to have a whole world map, there is no way you could fit the whole world into a game, not with any form of detail or realism anyway.
A good idea, which I heard was originally going to be in F3 is for it to be set in 1 large city, a very nice idea but then what of the world map? Well I have had the idea of making the game in one Large city, a city with so many diffrent places and such that it actually has its own worl map, in the center, for example, could be a huge crater (or the edge of one) that would be like the wastes. On one side you could have the main populated area, near the main souce of water or somthing, and in another area a raider camp or a goul encampment.
Alternatively you could emply Staillion travel like the wild west, just you, your gun and your horse traveling around. You could start with a Bramin, then have to work your way up to a horse. The more pristegious people in the larger places having trucks or cars (which you could 'aquire' but which take many more resources to use.).
DONT for the love of god make the world map a map of the world, it would NOT WORK! Have a small area, maybe not even as large as in F1 or 2, personally I though with all the walking the player did they would be able to kick Frank Horrigan in half at the end of it all.
Make the game area smaller but still be able to incorperate all the classic Fallout features like Raiders, random encounters, trading partys. I think the resusrected train line idea is a good one, having custom made trains being drawn by Horse or Bramin. Charging to take passengers and goods from city to city.
Intercontenental travel is a good idea (BTW you can solve the language barrier problem with the PIPBOY, its GOT to have an in built translator) but on a small scale, no globe treckers here please. Perhaps a boating system that takes the player between a few islands or even a continet or at most 2. That would be cool and the seasons idea is a good one, (Nice one the man who came up with the frozen lake concept!) In summer the player could require more water and in winter more food to keep warm.
Another well welcome addition to the game in my eyes would be charicter appearance customisation. ranging from face, weight, race to the cloths they (you) wear. One thing i did find annoying was the fact you could only wear pre set cloths. The player couldnt think to themselfs "hmmm... I wonder what the Metal boots will look like with the leather armour..." And things like that, More game customisation.