With a few exceptions of painters like Bosch and Bruegel you can safely just skip to the 19th century and not miss much. Pre 19th century can be summed up as portrait of some dude, portrait of some chick, landscape, greek god, and Jesus x 5000.
Trying to select more dramatic images to study- I want to focus more on dramatic lighting and color. So I started looking at classic landscape paintings (thanks for the tip, Jackwhat) Today I ripped off Cliffs of the Upper Colorado River by Thomas Moran
So the tiling is too big? Maybe try changing the Mapping Scale of the Terrain Coords in your Landscape material? Your terrain is the same color as your building - maybe try making the building a lighter color so it pops out :) But this is looking cool, I like the little lamps!
If they're large try something more along the lines of what they did in Ghost Recon: https://80.lv/articles/landscape-and-material-pipeline-of-ghost-recon-wildlands/ just custom bake will likely look rough on large things. Custom + tiling should work out well.
Makes sense that it would have similarities to LOTR/Hobbit as J.R.R Tolkien based a lot of the books settings on the changing landscape of Britain and its industrialization. Some of those are a bit far-fetched though. I absolutely loved the entire thing. Wouldn't be surprised if that gets Danny Boyle a Knighthood.
A heightmap's originally a specific case of displacement map used for landscapes, the one where all displacement is vertical based on a flat or spherical original terrain (thus heightmap, ie each texel specifies a terrain height). Displacement map are the general case with an arbitrary mesh and displacement along normals.
Not too sure if you really need to expand it much if at all, though I do like landscape/wide environment images. Just be sure to get something interesting poking into the bottom or top of the frame. A war room might be cool, a table(s), or holo display(s), etc...
Here I am positioning a modelled bunker in the level. So I can work out the correct scale. and make sure it fits in the level properly. I will tweak the landscape to fit the buildings nicely to mimic what weather erosion and time would have done naturally.
Houdini has some great landscape tools and you can import your height map and start with that.I don't think you can export textures with the free version but you can export your mesh. I'm interested in doing something similar in designer as well.
At first glance Cryengine sounds better, it has great outdoor lighting and day cycles, arguably better terrain and large scale landscape tools and systems, and seems to be designed around being more dynamic. But if you could get your hands on UE4 that'll be a different comparision.