I think the fact that you specify beautiful means that it will have to be done by hand and use references, to have the associated detail and design. But then the fact that you also specify huge means you’re probably going to have to use things like height maps at least at an early stage to block eveything out. So I think you will need to use both options and more (ie actual level design and how it effect flow and gameplay, something i know little about but is a huge area of study in itself).
I think one course of action would be:
- deconstruct a smaller area to see how it is built (here is where height maps could come in handy as a source material)
- build your own smaller areas that can later be combined in a modular sense
- once you have a few of these, start to look at how they would fit in a larger world, and look at combining them with perhaps sparser or different types of inbetween terrain.