I've spent the last ~5 years developing a terrain engine in this space (also on top of THREE.js), that you can find here: www.procedural.eu - it supports 3D models for houses and trees and roads, and uses procedural generation to augment the data. The approach taken if different from other engines, but is limited to a set area.

Recently I've also been looking at working on a terrain streaming version, a demo of which you can find here: http://next.procedural.eu.s3-website.eu-west-2.amazonaws.com... - this is lacking all the features of the full Procedural engine, but I'm pretty pleased with the performance. It looks like your engine only has tiles of a static size, which means that if you zoom out you lose terrain, and if you zoom in it is blurry.

Thanks for posting, great to see others working in this space!