I constantly fiddle around with my snap areas. The graphical interface to manipulate them is good and all, and bigger batch changes I can do in the JSON, but it would be nice with something in between. My proposal:
Add "Span Areas" as a category alongside the others (Trackpad, Keyboard Shortcuts, Normal Mouse, Touchbar, etc). List them in the same way, and have fields for the numbers in the right pane. There could also be buttons leading to graphical views, if preferred. It would also make it possible to move them between presets without having to bounce on JSONs, and you could even apply actions to them.
Here's an example mock-up:
The "px" dropdows would also have "%", and checkboxes for "Consider menu bar", "Consider dock", pixel offsets etc would probably be useful.
Colors, border width, corner radius and similar settings would of course also be in there, but there's a limit to how much effort I put into a mock-up. 
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