Author has written 5 stories for Ranma, Hellsing, Sailor Moon, DC Elseworlds, and Marvel.
I am a long time fan of cartoons, films, and anime regarding the supernatural and martial arts. As a result my fanfics will tend to be crossovers using what details exist to try and link them together. The thing is some have so many 'hooks' that my fan ficts do tend to become crossover bonazas. I also like alternate timeline/universe stories so that will tend to figure in my fanfics as well.
Perhaps it is because I have trained as an anthropologist but I have never bought into the whole 'It's magic therefore it doesn't make sense and-or doesn't need to be explained' excuse because even in our world those who believe in magic hold that it does have rules and laws. In those settings where magic actually and constantly works you would have a science of magic which the GURPS RPG gives the name Thaumaturgy.
While "GURPS 4e Fantasy" provides a general view regarding this, "Authentic Thaumaturgy" by Isaac Bonewits is a better source for this because it uses the concepts and beliefs regarding how magic supposedly works in *our* world as the baseline of how magic would work in a world where it could be produced on demand (and therefore studied). I would highly recommend both references to anyone writing a fanfic with working magic in it.
I would also recommend that authors keep a variant of Clarke's Law ('any significantly advanced *or different* technology is indistinguishable from magic') and Niven's Law ('any sufficiently rigorously defined magic is indistinguishable from technology') in mind with regards to working magic.