Sometimes we need to respond on new image derivate creation. Currently we can do this if we alter the image.style_public and image.style_private routes and replace Drupal\image\Controller\ImageStyleDownloadController::deliver with our own implementation.
I found that #2685905: Refactor ImageStyleDownloadController so derivatives can be generated by contrib modules is trying to solve this too but with a different approach.
If there would be a hook (image_derivative_created) we could replace most of these re-implementations with a simple hook implementation.
