Tenkar is facing a choice about what to do with his proposed campaign.
Having a bit of involvement in the boxed set Wilderlands as well as being the author of Blackmarsh and Southlands. This would be my "official" opinion on the matter of combining them.
Of course there is no desert along the northern edge of Map 9 (New Map 1) Elephand Lands but there are plenty of blank hexes east of the Land of Beasts to pencil one in. My Grand Kingdom of Blackmarsh/Southland doesn't exist in the Wilderlands but there is a rather large bit of white space to the east where it could be placed. Also I consider the Elephand lands to be the most DnDish of all the Wilderlands map as it is a polygot collection of village of different races in a forest land.
As a bonus you can see how I meant for Blackmarsh and Southland to be fit together.