Wish I had room and funds for that big of a tank.

I like the natural looking setups. I'm not sure on the type of fish. I am getting Cichlids as I like the colors of them.
What I would go with:
-Black gravel (bring out the color of the fish and surrounding, your main attention should be on them anyways)
-Variety of plants (I prefer live,as long as agreed with fish)
-Mix of Large/small stones (provide hiding and river/lake bed effect)
-Fish (I like more colorful fish, which is why I'm going with the Cichlids). Watch out, from what I've read with them as they can be quite aggressive, but with a larger tank should be ok with other types (Please correct me if I am wrong).
I like the slate? rock as you can prop it up an a smaller rock or stagger them on top of each other to give lots of hiding spaces and cool looking caves. I would then put some of the plants fake or living in between some of the rocks to give a more natural feel to it.
Just my 2cents worth. Maybe it will help
Good luck!