Okay, fanfiction -
I understand that it's fanfiction and that you are playing in an established world that has its own rules and canon, but please, please, please, label an AU as an AU. I cannot begin to elaborate on how stabby it makes me to read a story and get into it and 2/3 of the way through the character just busts out with something off the wall - like wings.
Sherlock Holmes does not have a uterus and therefor cannot be pregnant with Watson's lovechild.
John Watson is not an incubus.
They do not live in a spaceship called the Baker Street.
All of this is fine and could be quite entertaining if I know that it is an AU from the begining!
You... You.... Just broke my brain.
1) I really want to read that fanfic.
2) As an author I do sometimes wonder how much to "spoil" a story with warnings. I mean, you can stamp the label "AU" on it, but really, all
fanfic is AU, so that could mean anything from... the characters have a realistic adventure that they never have in canon, to Sherlock being a pirate captain while John is a merman crew member. And if a big part of your story is that, SHAZAM, halfway through magic comes into it, suddenly explaining all the weird things that were happening before, it can be tough to decide if you want to put that in the tags/summary/etc. or risk people being irritated when they finally see where you're going.
Thinking about the way I read stuff (fanfic and
books), I would probably rather know a few more details even at the risk of spoiling things, I guess. On the other hand my mom, who normally doesn't go in for fantasy/supernatural stuff, has been sucked into a couple of books without realizing they were going to turn genre on her, and she kept reading anyway and really enjoyed them. (Of course examples escape my memory at the moment.) She wouldn't have picked them up at all if she'd known where they were going, because she would say she's "not into that."