I want a subclass where one of the powers is literally accompanied by theme music.
The Summoner class I've been slowly chewing on for the past like...two years...has a subclass that could potentially fit the bill, once I write it. Its subclasses are called Astral Signs, signifying the hidden, secret constellations through which the Summoner contacts beings from extremely distant planes. The relevant subclass in this case is the Sign of the Muses, which focuses on supporting both the summoner's party and more specifically the planar creature the summoner is bonded to.
It's built off the Warlock chassis, so you have a split subclass, effectively you choose your mechanics at 1st level and your overall "theme" at 3rd (elemental, celestial, or umbral--for celestial, think "Biblically-accurate angels," and for umbral, think "the kind of plane where mindflayers are a perfectly ordinary part of the ecology," so basically the Far Realm or stuff like PF's kytons.) So I could probably have a Muses-specific "Invocation"* that involves literally having background music while you do things.
*I don't call them invocations. Trying to be cagey about terminology here.