In GTA 5. When you stole a plane or helicopter. At a certain altitude, nothing happens. Even on a 5 star wanted level, the cops cant even see you. And the clouds were just an effect that shrouded the map.


It's a big, mostly empty space for you to fly around. There's not really any depth to it.


This could completely change in GTA 6.
 

RDR 2 gave a small hint of what they were trying next with the cloud generation tech. 

GTA 6 will more than likely push weather technology to an insane degree. Heavy storms with lightning, wind and dark clouds, not only effecting visibility but your plane handling too.

On top of this, one can reasonably assume that we will actually see a lot of NPCs also populate this space. Other small planes, helicopters, passenger jets, other players (in online) and most importantly. 

Enemy planes and choppers. Drug cartels, military, police etc can probably actually give you a proper chase or fight this time around. They've clearly shown off the sheer amount of variety and density of air vehicles already. 

(Look at the very top right of this scene. There's a second jet flying right above the clouds. Which again, look MASSIVE)

The sky could very possibly even have its own random events and easter eggs (fingers crossed on an alien spaceship being visible in a storm briefly). 

And flight being more dynamic with different weather effects, wind, other NPCs and incredible visuals could essentially become a massive biome of it's own. 

The sky is going to be it's own massive biome