Animusic videos are nice concepts: depicting real synthesized sounds as if made by fictional musical instruments difficult or impossible to build in the real world. Additional ideas:
Even though the instruments are fictional, make them as realistic as possible within the rules of their fictional world. For example, playing the instrument the same way always results in the same sound; equivalently, if we hear a variation in an instrument's sound (needed for musical reasons), there must be a corresponding visual difference in the animation.
Let a mechanism to play an instrument require a solving a formidable puzzle, perhaps a constraint satisfaction problem, that no human could solve live if the instrument existed for real. Maybe balls bouncing off each other in complicated ways before finally hitting something to make a sound.
Depict the instrument in virtual reality, allowing the viewer to see it functioning from arbitrary angles and distances. Cutaway and adjusting lights. Rewind and slow motion.