Enhanced Flying Experience
As we all know, if you take a boat out to sea and drive in one direction, you will eventually reach a point where you appear as if you are progressing when in fact you are not. This gives the illusion that you are travelling further than you actually are.
What if we applied the same principle for airspace; considering that GTA V is a relatively small map and it does not take long to fly from coast to coast. Once you reach a specific altitude, you would pass the threshold at which point the mod will activate. And once you ascend past that point, it will take you 'x' amount of time to re-enter regular airspace, no matter how slowly or rapidly you descend.
This would be great because once you pass that threshold, you will be able to fly for hours without actually going anywhere and risk flying too far out in the ocean or away from the city. This would be great for plane mods, crash movies or whatever fun you can conjure up in the sky.