XenoSandboxGenerator

This Asset allows you to create your own
Sandbox Level with only one button click

It creates the Map by stitching multiple
Terrains together and then automaticly places
buildings, objects and vegetation on the Map
It also creates a map border automaticly

This package does also come with a custom Terrain shader with:
-distance blending (to minimize tiling)
-noise overlay (to minimize tiling)
-Triplanar mapping
-hard blending between textures

This package also contains sample object prefabs with lod models, ground textures and a sample scene.
for more information have a look here:
Article