I don't know how large a task this would be to the Blender core. It would be great if we had the ability to write custom modifiers that could be added to the existing modifier list. For instance, if I wanted to write a mirror modifier with additional functionality or a different workflow. This modifier would be added to the existing list of modifiers and used like any other.

I think the benefit to adding this would be an expansion of modifier tools and non destructive workflows.