What would be good in Outerra are rocky outcrops on areas with high slope show slightly different texture variations depending on the underlying geology.
This could be defined with a fairly low resolution raster (~ 1km as geology generally doesn't change over a small distance) and would only need a few classes to cover the range of different rock 'colours' that out-crop on steep slopes (i.e. tan mudstone, grey schist/greywacke, orange/brown sandstones in the grandcanyon, white/light grey limestone, white chalk)
This would avoid tying the rock types to specific land classes so you could re-use say, a pine landcover texture regardless of whether that pine forest was sitting on top mudstone or schist. It would also allow better definition of large open-pit mines.
I've just released a landclass/texture update for FSX for New Zealand and one of the things that 'would have been cool' was to properly map the underlying geology so that when the FSX scenery engine remapped the landcover texture to rock on steep slopes, that rock would change depending on where you were in the country...unfortunately it is not possible to do in FSX without seriously re-working the textures (Some screen shots if anyone is interested:
http://www.vectorlandclass.co.nz/index.php?option=com_content&view=article&id=53&Itemid=65) (I'd also say, don't be too pushed to get a demo of Outerra out if it means hacking stuff together just to get it to work in time