Mugen game design involves creating custom fighting games using the Mugen engine. With Mugen, designers can add new characters, stages, music, and special effects to create a unique gaming experience. The Mugen engine is highly customizable, allowing designers to adjust everything from the gameplay mechanics to the user interface. Designers can create their own storylines, develop their own characters, and even build their own fighting game franchises. Mugen game design is a highly creative process that requires a deep understanding of game mechanics and design principles.