Betty's theory of Fanfic
So, petronelle pointed me to thebratqueen's two step theory of fanfiction which is very thinky, and got me started. In essence, for those too lazy to click the link, TBQ argues that fanfic should stay within two steps of canon if you want your readers to come with you. They can be two big steps, but they need to be there.

1. Gotham is hit by a meteor and everyone dies
2. The ghosts of Batman and Robin haunt the site a million years later.

or

1. Bruce realizes he is in love with Alfred
2. Smut.

I may be misrepresenting her, so I encourage you to go read it yourselves.

However, to me, the way fanfic works is slightly different. I think fanfic, or at least a large body of fanfic works by asking, "what if?" What if Tim were transgendered? What if Bruce were a psychopath in the clinical sense? Or perhaps merely "if." If Bruce gave Tim homework, what would it look like? If Tim and Cass got to really know each other, what would happen?

Ifs are free. Your readers are willing to spot you your if; you don't have to justify it. Your if can be, "What if Bruce were a hamster," and your readers will go, "oooookay, lemme see what you've got." Where the writer earns the right to the if is how she handles the then.

If: Steph got a Green Lantern ring
Then: she would discover that power sometimes makes existing problems more difficult to solve, and brings its own problems with it.

The then is the hard part. Think of it as an equation: if + canon = then. You can't cheat on the then. The if is where all the cheating comes in. The if is your variable, the then is the result, and adjusting your results for a nicer hypothesis is frowned on the scientific fanfic community. Now, you can fiddle with your if until you get a then you like, that's certainly permitted, although to my mind the more difficult way to go about it. But you can't cheat on the then if you want your readers to trust you. You don't get:

if: Dick and Babs slept together due to wacky sex-pollen shenanigans
then: they live together happily ever after.

It doesn't follow, and besides, you've slipped something in there invisibly next to the if and the canon in the equation. That's cheating.

A successfully executed then earns you the right to another if, if you so desire. You don't get:

if: Bruce and Clark are in a sexual relationship
if: Kon and Tim are in a sexual relationship
if: Kara and Cass are in a sexual relationship
then: it's like the Brady Bunch but with more incest!

However:

if: Bruce and Clark are in a sexual relationship
then: their respective wards/half-clones find out, and work out how they feel about it.
if: Tim and Kon are forced to examine their feelings
then: they come to a new awareness of how they feel about each other.
if: Kara and Cass feel left out and start spending time with each other
then: it's like the Brady Bunch, but with more incest!

Could work, if the author was very patient, and willing to let it develop organically.

I think some fic takes canon up on the if it neglected to answer, and some fic merely posits an if and lets the reader supply the then. But that's another ass-load of blather for another time.

Argue with me!