In my opinion, including a feature like live editing inside a tool/feature can bring a good amount of convenience. Live editing like one can create or edit geometry while being inside a feature (I was a CATIA user before this and felt this feature in it quite convenient. Just had to right click on the respective selection windows and there we go with many options being displayed).
Although, I don't know if this point counts for UI, there is a good scope of upgradation for the construction tools. At present I feel there is a lack in the tools provided for construction. Again taking a reference, CATIA provides a tool by the name Extremum point which comes in my use a lot. And why only axis, why not lines? Not including construction tools for lines really makes one go through many indirect steps which could simply be avoided.
Also, there are instances where it feels 3D sketching with Fusion can improve. Like while inside a sketch, if we change the plane of sketch, wouldn't it be a better option to stay in the recent plane? That way we can easily define the sketch we make in that plane. Otherwise, currently when we stop using or switch from a tool while using 3D sketch, the working plane changes to the original plane of the sketch. This often leaves the sketch features undefined.
These were my thoughts on potential improvements. I love the minimalistic nature of Fusion 360, its interface and what Fusion offers.