Good base to start from. Some thoughts on potential improvements if you want to keep going (excluding adding new features like progression systems, fail/win con, enemy variety, etc):
1) Replace the upgrades that increase the wave duration (More enemies, ghost enemies) with more interesting upgrades. Upgrades should increase the player's ability to clear waves
2) Add more vertical vision. This will allow players to plan their moves better and hit enemies more effectively, making this game have a strategy element beyond just reaction times.
3) Add combos, and massively increase the distance enemies bounce away from you. These two changes together with #2 will make for a super satisfying gameplay loop where you need to bounce off of separate enemies to keep your combo going.