Creativity Through Context Replacement is the most effective tool for generating new ideas, and one of the best methods for doing so is the Mashup. At its core, a Mashup is a novel collision of two ideas from different contexts into a new shared context. Many popular creative innovations throughout history have been the result of taking two familiar but disparate ideas and combining them in a unique way.
Game Design Mashups
An example of this in Game Design is through Atomizing and Abstracting Game Mechanics of two or more games and recombining them into novel formulations. Once you successfully reduce a system down to its component design patterns, swapping out similar patterns is both trivial and extremely fruitful.
An example Raph Koster presents is not just experimenting with mashing up games with a shared underlying Topology, but to also Change the Topology of a Game by mashing it up with seemingly unrelated games with different underling topologies.
Practical Creativity by Raph Koster, GDC 2014 PKMashup Notes Tips for Creative Game Design