Hiho,
I worked on a variety of hills with different layouts and design.
What you have so far looks already like a good basis. Others gave you already very good advise, so I just add two other ideas:
You can use woodfiller and apply it generously to the rock face. After it is dry you can carve it and run a steel brush over it to depict stratifications. As you cna see I just used whatever pieces of XPS I still had around and combined them to a pleasing result.
Bark also works very well for some kinds of stone:
This one may be already a bit too naturalistic for your needs. You can not place a whole unit on it easily, but I wanted something dramatic

.
Obviously you can also go for a more stepped design:
This one I made with a cutter. I don't think it looks too realistic, but unit placement is super easy.
Finally, if you want to place trees etc. on your hill I recommend to integrate some steel tacks:
Then you can place trees that have a magnet in the bottom on the hill:
I wrote a tutorial on the magnetised hills, it may come in handy for your own endeavors:
http://wp.me/p49oPQ-F1