The creators of Spirit Tactic, a real-time tactics (RTT) indie title for mobile, have made a 3d modular terrain kit tutorial to help developers create modular terrain.
Designing Mobile-Friendly Modular Terrain
The list of 18 basic pieces are quite versatile and they all must conform to the base tile that was defined earlier.
The ramp lengths should be planned by base tile units which is dependent on the angle you want.
Snaps should be used to match the pieces that are at tile corners.
To get more variation without vertex blending, make mini-atlases with four variations. Duplicate a tileable texture and add details to it and map sets of four base tiles on it.
Add texture and map pieces so the ends match up and use a mini-atlas for detail. The tile should be scaled to 80% and trims should be added in the 20%. Use the same structure for multiple textures for the same tileset.
Try out different profiles with the same principles. The polycount will get larger the more tiles you have.
They imported it in Unity and used a shader that blends 2 textures by Vertex Alpha.