I disagree with many who assert that Rohan will or should look anything like Eregion. Eregion is definitely higher in elevation, MUCH different when it comes to vegetation, and as depicted in-game, is much too dry.
That said: Rohan is comprised of a few distinct regions, but let's focus for a minute on the wide steppe-plains of the west and east emnets.
This is ostensibly what it should look like:
Now, let's break the region down a bit. Unlike 'Southern Mirkwood' which has a few different 'zones', Rohan will have to be a collection of 'regions' akin to Eriador. Here is Rohan as we know it:
Here is Rohan as it could be divided into regions OF ROUGHLY EQUAL SIZE TO THE SoA REGIONS:
Let's go from the core out. -The smallest scale implementation of Rohan would include the West and East emnets as well as the Eastfold. -Anorien and the fens of the Entwash delta might wait until a Gondor expansion. -I expect Isengard to be a mini-region akin to Rivendell. -I believe it likely that we'll see Dunland not as part of the Rohan expac, but as part of a book update before or after. -The Wold is largely empty, and if they're going to skip any terrain here, I'd expect it to be that, sadly. A better alternative would be to lightly populate it and use iterative terrain to create a big open field. -Fangorn...whether its part of Rohan, or an update unto itself, I cannot foresee a future in which Fangorn is not included. -The Anduin, Emyn Muil, and Dead Marshes all might be part of a different update or storyline, but as we know them from the books, we tend to associate them with one another. I would hope at least, that we could visit Amon Hen as part of the East Emnet. Perhaps the Great River could be its own book update.