ok, lets give it a try.
ill post a starting tgd this evening with some basic terrain; after that, i would suggest the following:
as in this forum so many great ideas and expertise come together, lets say everyone can edit the file whenever he wants.
i think this will give the best result as we can help each other.
If you have an idea but don't know how to accomplish it, this way we can help each other.
but i suggest some rules:
- no premium presets, as everybody should be able (and allowed) te edit the file. and don't forget to give credits.
- same for objects; if you use any, please post it or a link where to find it.
- if you edit the file, try to use clear names for shaders or groups (most difficult for me!). i think the best would be to add a description of what you did in your post here. this way everybody can understand what what you did and learn from this.
- respect the work of others; don't just delete, edit if its a good idea.
apart from that, change everything and be creative.

i think this is a great possibility to a) make a stunning image and b) learn a lot from each other.
any ideas, additions or "what the heck are you talking about, lets make it completely different" welcome!