Extend on_gui_click by a position parameter, x and y relative to the clicked element. Not very useful for buttons etc, but for cameras there certainly are use cases.
Add on_gui_mouse_down, on_gui_mouse_up. Contains the same as on_gui_click + position.
Add on_gui_mouse_move. Contains element, player, position.
Add on_mouse_wheel. Contains the same as the above minus button. Obviously also contains wheel delta.
Please let me know what you think
