If you feel that you can't think of details to add, find actual photographs for reference. In order to add details, you first need to know what details to add.

I'm not super familiar with digital art mark-making, but I think that changing up the line width would help for you to get finer details, as well as create more variety and interest in your paintings. Instead of lining everything in the same thick, black, sketchy lines, try some more nuanced or delicate definition of shapes (you don't even need to rely on lines to outline shapes if you develop your shading more, although I do like the stylistic effect that you have here).

I don't know how I feel about simply superimposing your painting onto a photorealistic background, because it makes your work look amateurish, with an almost MS Paint vibe. Instead, try interspersing elements from the sourced image throughout your own composition (think of it almost like a collage on your painting). Also, creating more variety of textures will balance your work with the sourced image to create more harmony between the two, reducing the effect of it looking like scribbling on a photograph.

For color, just add more variety of the tones your already have in your painting — this will reduce any flatness, especially in the walls of the buildings that are currently just slabs of one color. Also, try to rely less on pure black and work instead with darker shades of the blue and green tones you have.

Compositionally, I mostly like what you have going here. I love the visual conflict between the figure and the blimp, although I think the figure should be sliiiightly more to the left (look up the rule of 3rds, if you are not already familiar with it). That round object near the bottom also draws my attention, but I don't think that's a good thing, since it pulls the viewer's eyes down and off the bottom of the painting.

Overall, I think you have a good concept and if you develop the elements noted above, you could have a notable piece.