Same problem as all fandoms: the toxic shippers that seem to go out of their way to ruin it for everyone else (and in extreme cases try to destroy other accounts, or even threaten an actor or actress, etc), and will find a way to push it (and strike at rivals, even possible rivals, to their ship) in about any thread, even when it had nothing to do with it (and then other toxic shippers come to fight them there and the actual thread is lost, destroyed by the shipping wars). That doesn't seem to be much of a problem here, though. (EDIT: not saying all shippers are toxic, the non-toxic ones don't bother me.)
My favorite is finding new and better headcanon as well as ways to interpret how magic works, the soul, the gypsy's end plan with that curse (putting a lot more thought into than the writers ever did), and sometimes group watches.
I've been awed by a few explanations (of character or how things work), rewrites, and even (I kid you not) calculating the time difference between Earth and Ken's Hell dimension so they could determine how long the portal that they climbed out of remained open (apparently about a single second on Earth, luckily time moved MUCH slower in Hell which is how they managed it).