ANOTHER REVISION: This story has a good amount of action in it. These people with powers are in an underground city because they are hunted by shadow government mercenaries who seek to keep them a secret from the world. There's a bit of a mystery with some murders in the city, too, but this isn't a mystery, so putting a dead body on the cover would give the wrong impression.
(in the floor plan of the city I've attached, the waterfall and cliff face are on the right side of the drawing)
I lost my description of the underground city, so I'll describe it again here, though I don't know if that'll help much. The city center is a football stadium sized domed area with a hole in the ceiling where sunlight shows through. The ceiling is a nearly lifelike painted sky, kind of like the Venetian Casino in Vegas, which means the hole in the center of the ceiling is supposed to be kind of like a sun. It beams in and hits roman numerals on the northern stone walkway that surrounds the park (the northern edge is kind of like a partial sundial with about eight roman numerals).
There's a circular center that takes up most of the area, which is a grassy park with a few trees, a gazebo, two Japanese wood bridges and one stone bridge at the eastern edge on the stone walkway surrounding the park. The bridges cross over a meandering river that goes through the center of the park. It's source is a waterfall coming from a cliff face that makes up a good portion of the western wall. The waterfall comes from a small opening at the top of the cliff, which is at the edge of the domed ceiling. There's a ridge at the edge of the domed ceiling all the way around from which light emanates upward, illuminating the painted sky ceiling. The little ridge makes it so you can't see the light source.
The outside circular wall of the area is flat stone with giant passageways (archways) that go into giant halls. The city was carved out by ancient giants. The rim of each archway has Ancient Paleo-Hebrew symbols (not old Biblical Hebrew, but the predecessor to that, the Paleo Hebrew).
There's a stone aqueduct about four feet above the ground that crosses from north to south. It comes out of the north wall and disappears into the south wall. The stone aqueduct is hoisted in the air by stone-carved tree trunks. It crosses over the river running through the city.
The outer rim surrounding the park is a stone walkway. There are equidistant spaced lights on the wall beside the walkway. The northern edge looks a little different than the rest of the park's walkway. You'll see that in the overhead floorplan of the central part of the city.
I only describe the city in case someone wants to take a crazy chance at making a really interesting illustration of the city. It could turn out good, I don't know.
REVISION: After seeing different designs, I realize that I do need a teen male and a teen female on the cover. The male should be a little more toward the front since he's the main character.
In a couple of days, I'll try to sketch out a little of the underground city, but you don't necessarily need to wait for that, especially if your design doesn't have anything to do with it. I need a full book cover, and I'll use the front cover portion for the e-book.