I think the way the movie handles magic, and the way it's a part of the girl's lives and their families, could make for some interesting fanfiction. Crossing over with already-magical series admittedly may be hard, but there's still potential, I think. For example, you could always have Sally and Gilly discovering that Melinda Warren was a great-great-great-however-many-greats-grandmother and they're related to the Charmed Ones and need The Power Of Five to defeat some Really Great Evil, if you really wanted to do a Charmed crossover.