The danger with these "very special fun episodes" is that they can be confined to being just that. But what elevated this episode is how it used the time travel/crossover conceit to foreshadow, progress and pay off SNW character arcs, including Chapel and Spock's ultimately doomed relationship (something that I've previously said could be incredibly poignant, if handled right), Number One's legacy, and the way Pike confronts his fate. I hope the musical episode does the same.