I started writing a Sailor Moon fanfic that was basically the magic system of my own devising being overlaid upon the Senshi and their villains.
Then it hit me. Why not make this a mostly-to-completely original work from the word go? The premise of the fic was that Usagi/Serena's power as Sailor Moon and her powers as the bearer of the Mark of the Guardian were both rooted in the love that she has for basically everybody, and therefore she was the only person available who was capable of showing the full power of not only the Mark of the Guardian but also the marks of the Healer and Universe as well.
I take out the Sailor Moon elements / reduce it to a couple of throwaway lines establishing the relative universe it occurs in, adjust the locality to one I'm familiar with, and I'll be good to go.