Based on user set rules (slope, height, etc). This would solve a major need.
edit: it seems to have some brush-texture tools that can paint with specific rules like slope, which are great, but a tool to go over the terrain and texture everything would still be quite viable and be a one-for-all solution to terrain texturing. plus more realistic and fitting for for big terrains.
Could use to texture your entire world, and than do some simple manual brush changes as you see fit.
check this out, should be coming out free:
“Rule-based terrain texturing tool”
Thanks, something official would be ever nicer though 
I’ll keep an eye on that one, if it releases.
But honestly this is something I expected from the terrain tools package, it’s critical for big terrains.
This is supposedly coming in the new “environment system”, first shown at Unite 2019.
Though it’s unclear what the current status is, as it seems to be held up by the development of splines and DOTS.
I’ve previously developed a tool for the same purpose here, though I agree a native solution would be welcome.