As odd as it sounds, I disagree with the requests to expand the Nentir Vale's world, at least in part. What I like about the Vale is that it is self-containted. It can be placed in any world you like. You can play an entire campaign within it without every having to leave.
As a compromise, I would be happy if they had an expanded world, but left any detailed reference to it out of any material published for the Nentir Vale. For instance in an adventure you meet an ambassador for "a sea fareing trade nation (like ABC, if you are playing in XYZ world)" without specific details a GM will later have to stick to.
Building on that: the Nentir Vale is a self-contained region, and is quite a lot of fun; so let's have
lots of other self-contained regions in the same vein, with no specified geographic relation to one another, so they can be mixed-and-matched at the DM's whim.
Of course a river implies a river-mouth; but we could have
several river-mouths, none of which need to be named "Nentir," but any of which could be used as the mouth of the Nentir in some people's campaigns. Some of the river-mouths could be estuaries or deltas like the Amazon or the Mississippi, while others could be steep, hilly bays like San Francisco or Seattle ("Deschutes Falls" at Tumwater, what?). Some river-mouths could feature a seasonally variable "bore" like the Severn, while others could have enormous religious significance like the Ganges.
We could have several deserts, several mountain-ranges, several desolate coasts, several archipelagos, several freeports, several muggy swamps, to give us variety to suit all tastes. Fanciful examples follow:
The Endless Steppes of Snarf!
The Pinnacles of Incorrigedes! (With the Foothills of Impassable.)
The Outback of TheyShootHorsesDon'tThey.
The Freeport of Pantacrumblo, where the unstable geology meets the ocean trench, and the land crumbles into the trench with every major earthquake -- reducing the high mountains to lower mountains, and forcing the population to shift the seaport roughly a dozen miles every fifty years. (Continual work for the civil engineers to rebuild the entire port and roadway system on an ongoing basis.)
The Lost Land of TooFarWest, where the Elves have been separated by vast oceans from the Silvan Elves for so many millennia that they still speak "Forlorn Elvish" instead of the more modern "Silvan Elvish." (No, wait; Tolkien already did that with Quenya. Nevermind.)