I'll go a bit meta on this: No, this is not a good idea. A good idea would be one that isn't obvious and defined well enough that one could start putting effort towards realizing it. That's not to say a more complex terrain generator would be a bad thing, but the idea as presented is about as good as "world peace" or "becoming rich and famous" - easy to say, but what is it worth without an idea how to approach it?
Even if double slopes were added to the current terrain generator, it would not cause "ultra mountaineous terain which deep canyons", it would just cause some of the slopes to be steep, rather randomly based on the noise used for map generation. Essentially, it would be the same as using the cloud-filter in photoshop (Filter>Render>Clouds) then add noise for the general hillyness (Filter>Noise>Add Noise) and import the result as a map in Simutrans. You can get about as good results as the ingame generator doing that, but it's not really what you would want.
So what does "more complex" mean?
As Vladki said, there were discussions about this already:
https://forum.simutrans.com/index.php/topic,17746.0.html (Started with map roughness)
https://forum.simutrans.com/index.php/topic,984.html (2008, mind you! Over ten years ago, before double height even existed, a more complex generator was something people wanted)
Furthermore, even if we discuss the details and come up with an actually good idea of how to approach it, it still needs someone to put the work in, which is the main reason there is not yet a better generator.
