If you want those adjacent tiles to look like something specific, the best place to start is mirror your terrain on that side and blending stuff in from there. Terrain Stitching / Blending - Using Gaea / Ask for Help - Gaea Community
If you don’t need the edges to look like anything in particular and just need something so the map doesn’t get cutoff into the void, you can just create a mesh of your terrain and mirror that on all sides.
Might not be feasible for a mod, but I use an RVT from my landscape’s material on this distant terrain mesh so the colors are an exact match. Here’s a picture of the grid and the edge of the map
