Any suggestions for what to add?
A variety of enemy and weapon types. You could do different characters that have different stats, or you could make it into a story based game, clearing areas on a map or something. Your game has the player clicking to attack, so giving the player a right click option may be nice.
Possibly add a map of there are objectives you'd want the player to complete.
You could try your hand at multiplayer (co-op or versus) if you're up for the task.
If you ever want help with planning, feel free to reach out (I definitely can't help with coding, but I can bounce ideas lol).