With Tools > Diff > Diff External File you can already compare two files.

Unfortunately, the foreground file selection window blocks Creator. I'm now doing Yocto development with Creator, where I need to diff files from deep nested directories.

For this case, it would be very convenient to diff two opened files (because open them with Locator is much easier).

So what I like to have is a further menu entry "Diff Open Files..." with a selection dialog that shows all open files where I can select two of them for comparing.