I like game as a concept. I hope you continue development. Here are a few suggestions:
- I think the game might be more enjoyable if you were able to move faster. I like that you have a base speed and a boost, but maybe the boost could be quite a bit faster than it is and then your plane could slowly ramp back down to the base speed, that way you can maintain some of your momentum from the boost.
- art and sounds, obviously
- The enemies spawn randomly. It's great that one will chase you every now and then, but maybe it would be worth having "airstrips" that the airborn enemies spawn from and "forts" for the land enemies, or something like that. Then they won't spawn in front of you, and you could have a bit more of a hand in deciding how to scale difficulty based on where the enemies spawn from, and how many there are.