Looks like retro MS DOS or 90s console game. I like it! Really! I am on the same idea as a developer with my game GALA.
Twice times i like your green coloured hud it is true about ms dos!!!
Behaviours.
1) Check GALAGA game. Old classic.
2) Enemies can try to fly on you and detonate near you. Asteroids can detonate after some time after they appeaer. They live 5 sec and explodes!
3) Myabe big slow enemies. With big sprite size.
4) auto piloted enemies on you.
5) enemies with diagonal moves.