Thats a nice game with interesting graphics and gameplay, good job!
I recommend:
- Animating the ghosts and giving them a ghosty/glowy shader
- Visualizing the ghost capturing, for example a backpack with a shader that shows how full it is.
- Different ghosts, and obstacles that makes the game harder for example water on the ground that makes the player fall and the ghosts escape the backpack.
- Final boss that tries to free his ghosts friends.
Just some ideas I got.
Well done :)