Certainly one of the big differences between fair use and fair dealing is the capacity, especially in the U.S. jurisprudence we've seen for fair use, to accommodate transformative uses writ large, so the real question that comes to the fore is whether a defendant's use of a particular work is simply a substitute for the original in the marketplace—one that simply appropriates the efforts and the originality of the underlying work—or whether the work is in fact itself a creative act, something that engages in another creative process and gives the public something new.
In my work, I've looked back historically to the origins of fair use to show that what concerned the British courts when it was evolving as an equitable doctrine was whether or not a new work was created with public benefit. In all of our attempts to create a fair dealing exception that creates sufficient space for ongoing creativity or for downstream creators, we're trying to find a way to ensure that a transformative use is not triggering copyright liability but rather is something that people are free to do. One way is to think about what we mean by a copy, and whether something really is just a colourable imitation of an original work or really is giving something new to the public.
This is something I would hope the courts would pick up on in part. It's something, however, that through the statute you could achieve by adding transformative use as a relevant purpose. Even within a fair dealing kind of provision, transformativity could be a purpose that leads to a fairness inquiry. Otherwise, of course, simply by moving to a fair use defence in which you're not limited by purposes but are in fact engaging in an assessment of the fairness of a use, you could entertain the same questions by asking whether there is new creativity, what the purpose of the use is and whether it's a substitute in the market for the original.