What aspects do you think make a great fan song for franchises like MLP and why?

Having a strong theme is key. Random EDM with pony samples can be cool, but having lyrics and musical styles that directly relate to the character brings it to a new level.

Rainbow Dash: Now, what have we learned?
Fluttershy: Loss of control.
Rainbow Dash: Good.
Fluttershy: Screaming and hollering.
Rainbow Dash: Yes, and most importantly?
Fluttershy: Passion!
Rainbow Dash: Right! So now that you know the elements of a good fan song, let's hear one.
Fluttershy: [inhales] Yay.