Learn more about animations (how to make a mech with different parts and play an animation on the end product in runtime)
Create multiple components of the same type for the mech with different functions and/or attack style(arms, legs, weapons, gadgets)
Use the necesary design patterns so that any robot could be used by player or AI.
Create an enemy factory that created procedural robots with randomized components.
Make an AI that could work and take advantage of any mech configuration.
Learn more about the UI in Unity.
Save and load the selected mechs.
Experiment with the twin stick shooter genre, chek if this was a good control scheme for the theme.