The mashup "genre" currently is full of amateurs, because copyright law won't allow you to make money off it. Sampling is used to great effect by hip hop artists, but only in very limited ways because they have to negotiate expensive licenses each time. If not for copyright, we'd see amazing innovations.

To get an idea what would be possible, listen to the Beastie Boys album "Paul's Boutique", which has been called "The Sgt. Pepper of Hip Hop". It liberally samples all kinds of sources (including Beatles), and the results are amazing, even with that limited 1989 technology. Unfortunately, shortly after that album was released, legal precedents around music copyright doomed any similar works until copyright law changes.