I don't know maybe someone already suggested this, but I still would like to suggest these two things related to GUI.
1. There's maintain option on some buildings, you can manipulate it's value using two buttons, + and -. If you will go higher than 500, value will become ∞(infinity). However if I want to make it much lower value like 0, 10, 50, you will need to decrease it by clicking - button many times. I suggest to make like if you click + even after ∞, value will reset to 0, so it won't waste our time.
And if you click - while value is 0, it will become ∞ or just max available value.
2. While navigating in building menu, you can click on category to enter it, but to leave it, you need to press arrow button, which also wastes our time. Solution would be to use right mouse button to leave category.
This two things will make us waste less our time and make GUI more user-friendly.