In several previous topics users express the desire for different types of targeted regeneration (as opposed to going over every singe image). One of the suggestions (target only featured images) in fact seems to have made it into the plugin.
I would say that more flexible targeting would be really helpful. For example not only featured images but how about everything BUT featured images? And probably more interestingly to regenerate by taxonomy? And even to add an option inside of any post edit page to only regenerate the media attached to that one post (that one is available in another plugin).