In my (swedish) everyday work these situations are the most common:

HD 16:9. 1920 x 1080 -> 1920 x 1080

HD 1:1,85 1920 x 1038 -> 1998 x 1080

HD 1:1,66 1793 x 1080 -> 1793 x 1080

HD 1:1,37 1480 x 1080 -> 1480 x 1080

HD 4:3 1436 x 1080 -> 1436 x 1080

HD scope. 1920 x 803 -> 2048 x 858

SD 4:3 768 x 576 -> 1436 x 1080

SD 16:9 letterboxed  768 x 431 -> 1920 x 1080

SD scope letterboxed 760 x 321 -> 2048 x 858 (nasty)

SD 16:9 anamorfic 768 x 576 -> 1920 x 1080

The numbers are calculated on my cellphone, please correct me if necessary.

Another alternative might be a checkbox "square pixels" combined with a sort of zoom or "fill aspect FLAT/SCOPE" function.

What I´m aimaing at is a scaling option that makes incoming picture fit into outgoing FLAT or SCOPE without any stretching or cropping. Stretching or cropping should be an active choice from the operator (assuming that he or she knows the consequences)

Some scope films has burned subtitles in the black bar. A "manual" or "custom" choice would be necessary to handle those.

/M.E.