I, unh, hope you like Xander or Faith, since most of my BtVS fanfic centers on those two, so let me apologize right up front if you're not a fan of either one. The good news is that you do have a little bit more wiggle room with regards to characters/stories you can grab. Not a lot of room, I know, but some.
The good news (if you dislike Xander or Faith) is that a lot of what I write is ensemble with a Xander-focus, feel free snag another character to focus on. Hopefully I've given you enough to work with if you decide to do that.
I threw together a compiled links list for everything I've got because it's scattered all over hell and creation. I haven't had a chance to go back an resolve all of the typos that need to be corrected on some of these stories (although I do plan to), so apologies in advance for that, too. Hopefully you'll find something you can work with within the list.
Also, I tend to err on the side of gen, so that (hopefully) gives you some wiggle room if you want to make something slightly more shippy than whatever I wrote. (Have I mentioned that I suck at ships? Because I really, really do. I think I'm missing the brain piece that actually allows other people to understand what "romance" is and how to write it.)
That said, I want you to have fun. Do whatever feels good and right to you. If you want to tackle a whole story, go to town. If you only want to remix a single chapter in a longer story (I tend to write novel-length) go for it. Grab whatever speaks to you and party it up.
Change as much as you want (I'm all for giving the remixer as much freedom as possible). Add new characters into the mix. Subtract characters that are there. Write an AU. Fiddle with the dialog to make it fit your version. Put together a "past fic" that leads up to the story. Put together a future fic to tell people what happens next. Or, hell, just write a different take on what's in front of you.
Make it fun for you. For me, half the fun of this exercise is seeing someone else have fun with something I wrote. If you're happy, than I'll be happy.
Thank you in advance for your hard work and lending your writing skills to the Remix project.