Just had a thought, maybe not original. Backdrops based on your own layout, rather than someone else's photos.
The concept is that you build a far-field scene for your own layout, using the topology, vegetation and/or buildings of your choice, then take a screenshot of it from a suitable distance. From that screenshot, you can create your own texture file to re-skin an existing backdrop asset. This might be especially useful for situations as described above by Falcus.
The far-field scene could take a bit of effort to set up, but it's only a temporary mock-up to be discarded after you have your screenshots, so you could afford to go nuts with the detail. The advantage is that you have total control of what is in the scene and it would be specific to your own layout, and even specific to a certain location in your layout. You would of course need to know how to crop and resize the image and how to add a suitable alpha channel to make the sky invisible, but that's very simple.
Are there any backdrop assets with a license allowing them to be re-skinned and released under the re-skinner's kuid number? If not, perhaps some public-spirited content creator could make some for this purpose.
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This sounds interesting. I have such a place that might work for me on one of my routes. I'm sure many of the already-created backdrops could be reskinned. They are meshes - just very thin ones...
John