It would be nice to have a copy of the layer that is dynamically updated. Rotation, scaling, and effects can be managed separately, but image content is locked and depends on the original.
Photoshop uses smart layeres, works from external files, but no such depths are required, just respond quickly in a document.
A particularly important feature is that there are many sub-units in the composition and something needs to be modified.
What features would you like to see in Pixelmator Pro?