Is it possible to extend Unity’s terrain painter with custom algorithms? Like sampling the terrain and calculating erosion. Or painting perlin noise on it.
You can read the terraindata and do whatever you like with it.
It’s just not very good documented how the terrain data works.
You would have to create a own script to do this, integrating into the original terrain tool from unity is probably not possible.
Do you know how to draw a circle on a terrain in the editor, so that you can at least create your own brush system?