Make a player character that can move around an environment and shoot projectiles.
Program some cool AI behaviors for various enemy game characters using C#.
Code a customizable enemy wave spawner.
Make a multi staged boss fight using state machine behaviors.
Make multiple weapons and power ups.
Learn how to make great main menus and responsive UI.
Create epic particle systems and sound effects.
Be able to make an entire game from A to Z using Unity, C# and some 2D application like Ps or Gimp. And then publish that finished product on Itch io, Newgrounds or Gamejolt.