Some ground based enemies / traps would be a nice addition to give you something else to look out for.
Besides variety, I think you need to balance the ghosts at the start, they are unfairly hard when they spawn on top of you. My final suggestion is to add some sort of obtainable goal. Like a certain number of ghosts in a limited time.
I reviewed this game on twitch, you can watch it here > https://www.twitch.tv/videos/790366622
=D