Hey there!

Zbrush is fun yes - just be careful trying to import a mesh of millions in to Max or Maya doesn't kill it! - I need to get back to Zbrush, been away from it too long.

Lot of video game related stuff works with zbrush max/maya work flows. Here's some environment Sculpting links that may interest you...

Various Tutorials Some of which also below.

Tiling Rock Wall Tutorial Console, Standard Development

Generic Wall Tutorial create high quality tiling meshes in a short amount of time.

Standard Materials Wood, Metal, Various Stones and Blocks

creating perfectly tiling meshes in Zbrush a workflow to create perfectly tiled normal maps on perfectly tiled lowpoly geometry, especially useful for cliff faces and rough stone surfaces.

Uncharted 2 Texture Workflow in ZBrush

How To Create Tiled Textures With Nonuniform Stones Using Zbrush


For importing stuff, there's a whole import section on the Outerra forum. people have been bringing in all sorts of things. I expect you've seen it

that link's cool you posted, thanks - clouds are cool too.

I suspecting you're aware of city Engine? another cool thing that would help make planetary scale stuff.

you’re probably aware of it but if not -

Esri CityEngine Trailer - YouTube

Esri CityEngine | 3D Modeling Software for Urban Environments

create procedural cities in no time

Examples - CityEngine and Unity - YouTube
Watch 1. CityEngine Basics | Procedural Inc. Episodes | Tech & Gadgets Videos | Blip
CityEngine Example Videos

people are using it for in game stuff

Unreal Game Engine featuring CityEngine - YouTube
Unreal Game Engine featuring CityEngine - Making Of

Examples - CityEngine and CryEngine - YouTube


sorry - a bit of a 'link dump'!!!

cheers
Jeremy