As a Roblox developer, it is currently impossible to load terrain on the client’s side.
This is a severe roadblock for one of my project’s development, as it revolves around loading levels/maps on the client side, but not on the server. Terrain can only be loaded by the server, hence it gets replicated to every client. If the terrain region is in a space that can be replicated, I believe you should be allowed to load terrain from the client (local scripts).
Terrain can be modified by the client anyways. In ROLVe’s Unit 1968 we’ve had issues with terrain server to client replication, and clients removing terrain for advantages. It gets treated just like BaseParts.