Another impractical accessory (No pun intended, but there is the booklet "Rolleiflex Practical Accessories") is the Rolleiphot iris attachment for the viewing lens, made in the 1930's. With it you could preview the depth of field on the ground glass. But it got so dark that you needed to use the accessory leather focusing hood. And it certainly didn't help that the Rolleiphot has a built-in grey filter that made the gg even darker. The reason for the grey filter was that you could use the Rolleiphot as an extinction light meter...
The Rolleiphot would have been more useful decades later, when the cameras got brighter screens. Perhaps for product photography in a well lit studio.