You should build a large mountain side pass with loads of secret entrances into the Mountain through chambers and such. That would be really, really awesome.
And by Mountain, I mean building it yourself, not using Slopes or another Map.
Also, you don't need to build like the whole mountain. Just really what looks like the side of a mountain and then little tunnels leading inside, so you don't even have to have the whole mountain full. It could just be a hollow casing.
Damn... now I want to build it. :/