A system I use now has the ability to select some images and then see two of them side-by-side. One is a primary that stays on screen, the others are 'candidates' that you can swipe through. You can change which is the primary.
The cool thing is that when you zoom into an image or scroll around it, the other image matches the actions.
Even if we could just select two images and do the scroll/zoom stuff, that would be fine.
I have to confess that my main use for this is pixel-peeping the difference between two similar images.
What features would you like to see in Pixelmator Photo?