Hello, I have completed the course and the expansions and I think it’s an excellent course! I learned much more about game design than I knew before, so thank you so much for making this course! 
I had two ideas for some expansion on this course. I’m going to try to implement them, but wanted to share those ideas.
Large Units - A unit that occupies 4 grid spaces (or 7 for hex).
Stairs/Ramps - Incremental grid height increases until the second floor is reached.
Please let me know what you think of these ideas!