This has been asked for a long, long while ago when the ability to paint on terrain at all was first possible and the limitations became apparent.
What we really need is a "visual" thumbnail choice of texture ability and ideally being able to choose to swap out a texture for another - any other - texture from hard disk location by direct access to a computer texture folders. i.e. to easily find and swap "any" painting textures for any level so that each level can use different ones and to the number and variety of permutations of individual level environments visually.
Same applies to other world objects in addition to Terrains that game makers should have the ability to easily change/swop textures on if not paint though the end result may be similar. i.e. change texture, materials and so on and be able to manage that internally. A pro system may have a texture/materials editor and so on though such things are expecting/asking a bit much given that at the moment we are not much further forward than a basic game making product with current features themselves needing updating to a more completed end user condition which is what is being asked for.
As others have done I am sure I had looked early at how users could possibly enhance the current scenes by attempting to creative use the limited feature we have and with effort one can probably achieve something a little better than what the default may provide by new texture design and level design use in applying them. At the end of the day though the limit is the limit and simply not enough to provide much more variety visually than it imposes.
Attached is a screen shot I output a long while back which attempts to show a range of terrain height levels as it were by placing snow on the hill tops and different textures on lower terrain. With variations in use of hard edged and overlay of textures and transparency effects one can seemingly extend the apparent number of textures somewhat though all graduations are of course limited to the same small number of textures. You just don't have enough for most levels varied requirement.
A few more would not go amiss and would be very helpful in users making much better looking levels both constructively (terrain features) and visually.