I am currently in the midst of creating terrain textures, and like in real life the terrain has different stratas. Think of how there could be grass on land, then some natural beach followed by a seabed that change with distance from the shore and the depth. Just to get the idea, here's an image.
In Sampler we can create beautiful terrain materials, but it's limited in that one isn't able to preview how they come together making it very hard to "marry them" together, which then requires lots and lots of exporting, trial and error.
I'd like to be able to preview two or more textures next to each other on the same mesh in the 3D viewport, perhaps controlled with something like the typical UX of gradient stops. Think perhaps, that you have a ruler-ish interface where you can add stops by hitting a + button, then clicking such a stop to choose a material (from the ones in the project file) for the partition, then you can slide the stop to decide on how big of a portion of the mesh it should take up.