Instead of gushing more about World Con, since my cohosts pretty much have that covered (eeee I got to have coffee with Tamora Pierce!!!), I'm going to talk about fanfiction. It's been on my mind a lot lately, since my roommate is writing pretty great Voltron Legendary Defender fanfic right now. We discussed it a little bit with Susan Spann on an episode a while ago, but that was more on the legal side, not on the fun side.
Like probably a lot of people, I got my start writing with fanfic. I wanted to see my favorite characters on other adventures, and I'm not ashamed to say I had some pretty intense self-insert characters for them to interact with, too. And, you know, untenable, crazy ideas of smashing various fandoms together. Seriously, I once tried to write a fic where the Stargate opened up to all of the other worlds I loved and a whole bunch of my favorite characters at the time joined the SGC. Because why not throw Drizzt d'Orden, Edward Elric, Aragorn, and and some random, obscure characters together? (Hint: too many characters, not enough plot. That's why.)
I think fanfic gets kind of a bad rap. Legal issues aside, it's a fun way to explore writing, and you're more likely to get feedback on a fanfic than an original story, depending on where you post it, which can help you improve. Plus, you can get a handle on small parts of writing craft one by one, since you don't need to worry about worldbuilding and character development right out of the gate.
The major thing to be aware of with fanfic is that these are not your characters. Be respectful of the original creator's vision and intention and if an author has asked for no fan works to be created of their world, for the love of all that's holy, listen to them. They created their world for people to enjoy, to tell their own story as a creator, not for people to trample through it and tell them how it's supposed to be run. Respect the creator's wishes and ideas, don't be a jerkface, and have fun!