I had an idea that could improve the visual quality of displacement mapping in Blender.
The gist of the idea is as follows:
The mesh is displaced as it normally is, but then detail is added with bump-mapping based on a modified version of the original displacement texture.
This would allow for apparently higher-detail displacement without increasing the mesh resolution.
For those of you who would like to know what I mean by "a modified version of the original displacement texture", here is my explanation:
The original texture would be modified by subtracting the actual displacement of the model's vertices (linear interpolation would be used between vertices to account for polygon surfaces). The resulting texture would be used as a bump-map on the model's surface.
This should, obviously, be an option that can be switched on and off, because it wouldn't be good for all situations. But for textures with a combination of large, broad displacements and small, detail displacements (such as the procedural clouds texture) this could be extremely useful.
This can already be done by hand by using a combination of displacement-maps and bump-maps, but it would be nice to automate it.