Daggerfall Unity is an open source recreation of Daggerfall in the Unity engine, adding quality of life features and mod support.
Here's another streaming terrain video where I go for a quick run from Daggerfall city down to the coast.
This just uses Unity's standard terrain system, augmented by my own streaming terrain manager, Daggerfall API, and custom shaders (for terrain texturing and billboards).
Everything is created procedurally at runtime from native data, and it's already possible to replace trees with models and add other details using Unity's standard feature-set. I will eventually add more options around this as the toolkit expands.