Thank you, Jamestown. This helps to reduce the possibilities.
The first idea that comes to mind is of course something that is part of most landscape images/panoramas, but perhaps not really seen as “landscape”: the sky, and with it the atmosphere it sets. Besides that, the time of the day plays certainly a role in the final result. It helps to get an initial direction for the whole mood. Use the Sky Object and the provided presets. It will certainly help.
The Grass Shader should be used more for close by areas, anything beyond might end up in long render-times. For anything a little bit more far off, check the Fur or Hair Object. Also here, if you render it out from specific points, you might get away (with the same image-quality) if you use Camera Projection. The rendering would help here to manipulate the rendering in Photoshop a little bit. (paint in it, or place stones and stuff into it.)
The geography itself can be taken from DEM data bases, there is even a plug in available, that allows for a fast creation. Have a look here:
http://www.cineversity.com/forums/viewthread/1397/
Besides that, you could use the Landscape object and place many of them, with different settings, in your “field of view” or what the camera will see later on.
Weathering Shader, Pavement Shader, Colorizer Shader, Layer Shader, Filter Shader, Gradient Shader, Noise Shader, Subsurface Scattering Shader, Falloff Shader, Mabel Shader, Nukei Shader, Terrain Mask, and perhaps some other shaders are the essential tool kit to create out of the Landscape Object more or less nice mountains, especially with the use of Sub-Polygon-Displacement.
You might see with the collection of shaders, the variety is already huge.
Besides that, you might as well use the Sculpting tool in C4D, to change and transform any object into landscape objects. If the amount of pixels exceeds after a while the power of the installed system, you might bake those to lower levels. have a look here:
http://www.cineversity.com/forums/viewthread/1409/
IF the topology is set, perhaps you take some images from trees and place those on Plane Objects. With that you use the MoGraph Cloner to populate the surface with trees and other plants. I talk here about 2D trees, especially for distant objects with a Target Effector, during set up. For trees closer to the camera, you need more detail. Very close trees often have a high demand on details before they look kind of believable. There are many options for trees for C4D. Onyx Tree might the oldest with two+ decades, X-Frog and other providers are frequently mentioned on the MAXON web-site, if there are some news about.
After all the option to camera project landscapes to a small piece of geometry, as discussed in my JET series is always possible.
Use the Fog Shader of the Environment object, to get some depth perspective into the scene.
As I get not tired to suggest this, here we go again: Get reference images, lots of. Collage those perhaps to something that comes close to your target. It might help tremendously to get clear about your targets and helps to compare your results with what you had intended to do.
For some parts, you might use BodyPaint3D as well.
Again, Vue has even a free version, I didn’t suggested it, as you used the term realistic. Since you cleared that, here you go: http://www.cornucopia3d.com/products/vue/vue_2014_pioneer/index.php
You can create your ideas there, and perhaps render out images that you project then inside of CINEMA 4D. I have done it with Vue XStream in that way for a movie that was printed on 35mm cinemascope. It is a fast technique and allows certainly for teh background for impressive images. Have a look.
My best wishes
Sassi