What I am trying to say is to have automatic fog of war lifted rather than having to manually fly a unit over enemy territory. Since HOI4 has individual planes rather than units having a small fraction of the planes flying reconnaissance missions is possible. This could be an passive (rather than active) lifting of fog of war just like radar stations in hearts of iron3 passively lifted fog of war deep into enemy territory.
Please pardon the writing on the pictures since they were for internal use in an old multiplayer game.
This is from an old multiplayer game in HOI3 notice the area surrounding the UK can been seen. With radar station upgrades the area that can been seen is extended. Photo reconnaissance missions could function the same way as radar upgrades in that they extend how far away you can see into the ocean/enemy territory.
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The next picture is from a different game and two years earlier so England has no radar upgrades so the area that can be seen into the ocean is less. So my proposal is without photo reconasance missions the areas that can be seen into oceans/enemy territory is less.
You will get a bonus if you got air superiority over a region.
Earlier games, airplanes did remove fog of war as they passed over regions.