Other rules that distinguish the fairy tale from other stories are more difficult to discern; many come under the heading of "I know them when I see them." J. R. R. Tolkien's Lord of the Rings trilogy smells of fairy tale through and through, as does his novella Smith of Wootton Major; the same author's Farmer Giles of Ham does not, even though it contains such common fairy-tale elements as a magical sword, a giant, and a dragon.
The fairy tale as a basis for more modern fantasies has a long and honorable history, ranging from Andre Norton's Year of the Unicorn (loosely based on "Beauty and the Beast") to Mercedes Lackey's Five Hundred Kingdoms series and Naomi Novik's Spinning Silver (a tale derived from "Rumpelstiltskin"). I'd like to add to that, but the challenge will be spinning a fresh story within the traditions and tropes that define the world of fairy tales. Creating something both old and new is certainly going to stretch my writing ability; I'm hoping it will be in a good direction.