I'd really like to have a menu system where I can select things along the line of "Maps->Tiles->Stone->[Variations of Stone Tiles]" or "Music->Fantasy->Battle->[Selection of Music]" and have it integrated with the Marketplace, allowing for streamlined creation with ease along the lines of LittleBigPlanet (which streamlined creation very well, IMO).
Maybe add a setting to allow which store you're viewing; The Free store, the Paid store, or your personal library.
The GM creation tools are currently fairly powerful, but not simple, intuitive or (frankly) fleshed-out enough to be able to provide the streamlined experience I expect from a current-gen toolset.