Want a lossless jpeg editor
Photographers constantly debate about jpeg. Should they shoot in RAW or JPEG, for example. RAW preserves everything, but is much harder and bulkier to work with, so you will see serious pro photographers, who you think would always vote for the "never throw away" logic of RAW, tell you they work mostly in JPEG. I'm one of them. I use raw only for shots with high dynamic range, like night photography, and often shoot RAW+JPEG to work with the JPEGs and pull the RAWs if you need it.
If you do work with JPEG, you still want to avoid editing with jpeg -- loading, changing and recompressing. Most people go to TIFF at that point, or PNG, because for the few photos you will actually work on, the space issue is minor, and TIFF can be used in almost all software that uses JPEG.
There are a variety of tools that will do a lossless rotate of a JPEG. If you have a JPEG that was shot in portrait mode, they will rotate the picture and create a new jpeg without loss of information. That's because JPEG compression breaks your photo up into little 8x8 blocks and loses data within those blocks. However, you can rotate those blocks at no loss in a picture which, like most, has dimensions that are multiples of 8.
Still, sometimes there is a temptation to do other edits on a file you have in jpeg, such as crops, touch-ups with the clone or healing tool and such. To assist that, a photo editor could support mostly-lossless jpeg editing. If, when saving a picture back, any particular 8x8 block of pixels has not changed since the original, write it back exactly as it was. For bonus points, handle rotating of such blocks, too. For other blocks, you must recompress, though you could arrange to always recompress at a quality level which will provide minimal loss.
Strictly, this would require crops to be on block boundaries. I think people might tolerate that. Alternately, one could do a special crop which creates an image with a small white or black border to allow the crop lines to be anywhere. If the user insists on cropping out those, the crop will no longer be lossless, and they should switch to tiff or png.
This requires a photo editor that is aware of the jpeg structure behind a photo, so it may not be trivial. But it would be handy.