Forest? Snowland? (http://en.wikipedia.org/wiki/Deciduous and http://en.wikipedia.org/wiki/Taiga)
I don't know how it looks like in that area, but some northern parts could be colder and have more trees (living or dead.. I remember Fallout 1 and 2 having forests... but kind of dead, or atleast lacking much leaves.)
Yeah I was thinking about having some forests and snowland up to the north and on the top of mountainous areas (I will add more soon).
I was also considering adding the area called the Deadlands to my map, if you don't know the Deadlands are:
'Land in what used to be Canada. Nothing has ever been found alive here by the MWBoS's scouts, just ashs for miles.'
-My Official Description
I was thinking this could set up for a DLC about a group of people who are living in a Vault and staying there due to the odd chemical a nearby lab was testing Pre-War which radiation mutated and made it so nothing could live or grow there.
The scientists could still be alive as mutants similar to the Master (spliced together in as a glob of flesh) who are allied with Davisom (that guy who leads those people who worship that brahmin skull, they think that this is their god's true form and the father of the Master) and his nightkin. Davison tells them of the Master's Army and they decide to do the same, abducting Vault Dwellers. Your character recieves a distress signal from the Vault and can head north with some MWBoS scouts who have also recieved the signal to see what's going on (doesn't matter what your reputation with the MWBoS is, these guys will be neutral).
You can choose to join Antler (the name the scientists have taken due to Davison and his Nightkin continuasly calling them that) and become mutated and watch ending slides of the MW being dominated by a 'New Master' or you can help the Vault Dwellers, kill Antler and watch as a few of Davison's nightkin escape to take vengance, then you lead the vault dwellers to safety in the MW.
That'd be an awesome DLC, it should be called Deadlands (like the area it takes place in).