Neph, regarding Gary:
My philosophy of fanfiction:Creativity means creating a good, original work. The goodness part is complicated. Originality isn't easy, but it is simple: Just don't do stuff that's already been done.
The First Law of Fanfiction states that every change which strengthens the protagonists requires a corresponding worsening of their challenges. Or in plainer language: You can't make Frodo a Jedi without giving Sauron the Death Star. Read any book on writing ever and it will tell you that stories are about conflict; a hero too strong for their conflict is no longer in tense, heart-pounding difficulty. For example, Dark!Dumbledore and Dark!Harry both permit a Harry strengthened over canon - the first by turning one of Harry's canon!allies against him, and the second by turning Harry against his canon!allies. The most spectacular application of this principle that I've seen is Harry Potter and the Wastelands of Time, in which Harry has gained all the knowledge of ancient Atlantis and has been through literally hundreds of Peggy Sue cycles in which he learns every possible twist of fate... and Voldemort, who unfortunately got to Atlantis first, has still won every time. The Mary Sue is not defined by her power, but by her lack of an even more powerful opponent. I mention this (1) so that you know I know it and (2) because the First Law of Fanfiction ought to be in a giant banner on every fanfiction site. The most fatal temptation of fanfiction writing is to think of how much easier some character's life would be if they were a ninja. We are naturally inclined to think up ways to solve our characters' problems for them, but must learn instead to make their lives more difficult.
The Rationalist Fanfiction Principle states that rationality is not magic; being rational does not require magical potential or royal bloodlines or even amazing gadgets, and the principles of rationality work for understandable reasons. A rationalist!hero should excel by thinking - moreover, thinking in understandable patterns that readers can, in principle, adopt for themselves. As opposed to the hero just being a born "genius" who comes up with amazing gadgets through an opaque discovery process, or who pulls off incredibly complicated gambits that would fail miserably if the reader tried something similar in real life. (Albeit that if, for some reason, you want to depict an eleven-year-old knowing all that rationality stuff, he must be extremely bright for his age level, which in turn has further implications. And no matter how bright he is, he'll still have problems - but problems for the hero are always a good thing, right?)
All fanfiction involves borrowing the original author's characters, situations, and world. It is ridiculous to turn around and complain if your own ideas get borrowed in turn. Anyone is welcome to steal any ideas from any fanfiction I write.
I have joined the Review Semi-Revolution: I resolve to leave at least one review on any fic of which I've read 15,000 words, regardless of the number of previous reviews, its age, or whether I have anything wittier to say than "Nice fic". On the theory that if I like it enough to read 15,000 words, I like it enough to leave at least one review. (This is halfway between the Review Revolution and the Review Counter-Revolution.)
If you enjoy reading a fic, that's easy to repay; just leave a review - reviews make authors happy. But what if you learn something from a fic, what what if it changes your mind? A favor like that isn't easy to repay, but what can't be paid back can often be paid onward. Blog the fic or tweet it or tell your friends, and someone else will get a chance to learn.