Compelling peice! with the light source to the right you might consider stronger shadows to sort of anchor the man and car more and help avoid the copy and paste look (unless this is the look you are wanting, which could very well be)
maybe grunge the guy up some. Love the colors and overall look, well thought out and executed. With some attention to details it could all come together
As Susi has said , the lighting , it will make or break an image as your brain will interpret if its believable or not without you even knowing it , when the light is wrong it will not look right no matter what ,,,,, your image is wrong in many areas of the lighting as each composite being shot different has its own inherent lighting , each from a different source , your job is mesh them all into one with your light source being the sun .
You can create the illusion of light by adhering to its known qualities , falloff over distance , angle of the surface struck in relation to the source which creates the bounced light , keep these in mind as you create your shadows in terms of where and what strength and color the shadows are .
The focal point of your image is the man " I think " you can get more impact by manipulate the lighting to lead the eye to him , like not stopping the lightbeam at the building but on him , lowering the brightness of the car in relation to him , by pushing everything back you bring him forward .
The buildings in the background should be blured more and get more so the futher in the distance they go , this is called Ariel Perspective which kinda adds noise and changes color over distance , the airship is the same deal, its to sharp and clean to be where it is in real life in relation to everthing else .
You idea is a good one and its only the lighting that lets it down , I suggest looking at Matte Painting forums as those guys are experts at this kinda of thing , the subject is a must understand as you create your art and its relative to all images , landscapes cityscapes even more so .
" you can get more impact by manipulate the lighting to lead the eye to him , like not stopping the lightbeam at the building but on him , lowering the brightness of the car in relation to him , by pushing everything back you bring him forward .