I know that you can go and change source options from the mod matrix, but having the ability to visually move mod sources around would be nice, e.g:
Alt+Drag - Move one mod circle
Alt+Shift+Drag - Move all mod circles in source
Alt+Ctrl+Drag - Move all mod circles you move your mouse over
This would be usefully when you have multiple modulations from a single source and want to quickly move them around.