Kusutto, you can use anything you wish, 3d/2d real life photos... Just have it depict your ideas on how "under" should look.
Remember this is concept on how these classic maps would look, so far nothing is worth using as concept art. Sorry, we need amazing photos or drawings of what these maps should resemble.
Color them, pencil sketches are nice to start a map from, but not to model around.
This could of been a Red alert concept map or something...we could model the buildings and bridge to match the art work.
In this example, we could use the jungle theme for maybe a map like under?
and this one we could use the theme of dead dinosaurs and huts on a map like field.
Finally there is a ton of real world photography we can use as a theme for our levels, for example, this could represent a look for canyon
or city flying with an middle east feel to it
So you dont have to be an artist to make something that suits your imagination of what you think a classic map should look like, make a collage of photos or draw or block out something in 3d, it doesnt matter how, just have something that catches the eye.
Also, gameplay must stay as close as possible, example, city flying will still have the same paths and large bridge, but it could be more room inside the tunnels or the bridge could be damaged or have ice hanging from it as if it's winter.
Hope I'm clear on what we like to see with this contest.