I'm pretty sure that this game is long forgotten by the dev, but if any updates are ever made it would be nice to have variation in the sky: clouds, birds, planes etc. Maybe even just different colours in the background depending on how high you are.
Many more acheievements would be nice, too. Getting all but the last one is simple enough, but the last one is insanely hard.
With this in mind, an upgrade which speeds up time once you've used all your boosters would be nice.
Another thing that might be fun is a counter of how many times the car has spun on a jump.
And finally a personal scoreboard showing your farthest distance, maximum height, furthest bounce, fastest speed, most spins etc.
Fun game, but at 3.4km height and 14km furthest jump, I am getting a bit bored!