Since we have the ability to access files, nice addition will be if we can browse images in full screen, swapping left or right, and flagging up and down.
Than selecting flagged images and send them into destination album in Photos library.
What features would you like to see in Pixelmator Photo?
Unfortunately, image previews aren't available in the Files-based browser that Pixelmator Pro is using. There are different view options (like the column view, for instance) you could try switching to if you need to see bigger versions of your photos.