It would be nice to have a feature for specifying two strings, one in from-file and one in to-file, which should be considered to match. Thus, if the two strings are ‘foo’ and ‘bar’, then if two lines differ only in that ‘foo’ in file 1 corresponds to ‘bar’ in file 2, the lines are treated as identical.
It is not clear how general this feature can or should be, or what syntax should be used for it.
A partial substitute is to filter one or both files before comparing, e.g.:
sed 's/foo/bar/g' file1 | diff - file2
However, this outputs the filtered text, not the original.