Where exactly do these taxis go? I'd remove the cloud plane, you really can't see much of anything below it, crop the image some, to right above the farthest back taxi, you don't need all that back expanse....jmo

yeah, it's missing some kind of focal point. Maybe if you had the towers of some massive city poking through the clouds way way off in the distance, or even miles down below the taxi rank?

I am in agreement about the emptiness in the scene... what you have is good... but a city in the distance would add a lot... as would some mountain or trees or some cacti in a sandy desert environment. Search the freestuff for Dystopia..... by Mobius. Very nice ready made cities you can incorporate into your image. Good start though. Those taxies are pretty neat. 8)