^ I used to write Sonic the Hedgehog and CatDog fanfiction. (No crossovers, though. And definitely no slash or erotica.) Like you, I also made friends within these communities, and I got a lot of feedback on my writing. (Mostly negative with the Sonic ones, a bit more positive feedback with the CatDog ones.) That was years and years ago, back in my teens and early 20s. I was thin-skinned and took the negative feedback poorly, but later on, I realized that if I wanted people to like my writing, I had to be more action-based. Less talky, more walky. Or whatever "genius-sounding" phrase I'd come up with back then.
My most recent venture into fanfiction happened a few years ago with a handful of Adventures of Sonic the Hedgehog short shorts/flash fiction/drabbles. I wanted to continue them, since AoStH is such a fun world, but after that experiment, I started to work on an original sci fi novel, and I never looked back. (First draft: ~140K. Second draft: ~230K. It's now resting while I work on other, shorter projects. No, I'm sorry, this particular novel is going to have to be that long, considering that I'm dealing with space travel, an alien society, a movie crew, and a storm from hell.) I wish I could write a few more drabbles, but then I think, "My other writing won't get done."
Writing fanfic was good practice, but I find more freedom and joy in creating my own worlds and characters. I've wanted to be a writer since I was 8, and while I did create my own stories as a child, there was something more comfortable about fanfiction. There were already established worlds and characters to play with. Of course, this being the 90s, they were pretty simplistic characters--cut-outs, even--so it was easier to keep track of them. I occasionally tweaked their traits to suit my purposes, of course, but I didn't do anything too dramatic. At least, as far as I can remember. Naturally, I made OCs, wrote stories about them... and was called boring for them. Oh, well.
I have some fond memories of those days, but like I said, there's more freedom and joy in my own works. Fanfiction was just a springboard.
And while we're on the subject of established writers doing fanfics, I'd like to point out that Cassandra Clare, author of the Mortal Instruments series--and all the flippin' spin offs she's planning--was a Big Name Fan in the Harry Potter fandom. In fact, the MI title itself and the plot was lifted from one of her HP fics (a lot of her detractors have stated that Clary = Ginny, Jace = Draco, Simon = Harry, etc.). There are also bits from her "Draco in Leather Pants" trilogy that were used in the MI books. If you Google long enough, you might find the whole trilogy in a handy zip file. Warning: the third book is a doozy.
