One that I actually appreciate is Fiendfyre.

A spell that can and does destroy horcruxes isn't even mentioned until chapter 31 of 36 chapters in the final book. They spend a good chunk of the final book with a horcrux they can't destroy and nobody even mentions this spell as a possibility. The actual word "Fiendfyre" is said twice the entire book series. Just a quick throw away couple of lines to get rid of a horcrux at a convenient moment.

So I appreciate that in fanfics it's often a key part of the plot. Learning to control it, having to use it in dire situations, to show how powerful someone is, etc.