One of my favorite mod in Skylines that you can import _real world_ terrain maps into the game. Building your own city but better is a good fun (and challenge!) https://terrain.party/ Hope this will work in the sequel too
I spent so many hours hunting for cool bits of real world terrain and then massaging them into playable scenarios for myself (it's a lot of work to smooth out artifacts in the data and then add functioning water, vegetation, initial road connections, etc.). I don't know what the licensing concerns for the elevation data and maps would be, but I hope they consider integrating this functionality directly into the game in the future.
You can get heightmaps from NASA's SRTM data without any licensing concerns at all, and they can be used in Cities Skylines (without mods) without any problem (provided you put the heightmap into the right file format). This was pretty much always possible, it's just been a lot of steps and sort of a hassle without projects like that facilitating the process for people.
It would be incredible to have a globe viewer/DEM download tool built in to the game, but the bandwith required for hosting the project might turn into an issue (I'm reminded of the situation with high res imagery for Command: Modern Operations/TacView, where users are strongly encouraged to download only what they need due to server constraints).
If you could find heightmaps dating back to seattle's founding that would be pretty annoying map, but only if you couldn't just flatten out parts of it at will with no cost. it's still pretty challenging geographically to this day, but it went through many re-grade projects as it grew.
