The key to keep in mind is that all space needs to be maximized and used most efficiently in a nano. I would start by looking at different tanks and deciding what types of features you like. Do you want as much free sand bed as possible? An island? A pillar? Arches? Or just a single structure with good viewing angles?
Once you know what kind of features you want, pick out rock judiciously. Try to choose individual rocks at a LFS that you can imagine being shaped into the structures you want. Once you got your rock, draw it out. Then try to build it outside the tank. When you are happy, try building it back inside your tank. Good luck.