I mention Tim and his RPG's because the idea I had is a follow-on from my fanfic, The Sisters of Paradox, and he adopted elements of that for RPG use.
(Just an aside - Would you believe I still get emails every now and then from people favoriting Sisters of Paradox on FanFiction.net?)
The story would have been called The Daughter of Paradox. It opens a couple of years after the end of Season 7, with Willow alone and depressed about the way her life has turned out. She goes into research mode and discovers a way to contact Faction Paradox, the time-traveling voodoo cult that altered time and prevented the birth of the original Willow (i.e. the Riff Regan version), thus enabling the Willow we all know and love to be born. She makes a startling request - she wants Faction Paradox to send her into an alternate timeline, one in which her spell to resurrect Buffy never worked. She's decided that bringing Buffy back was a mistake. Buffy had been happy in whatever heavenly dimension she'd gone to. If the spell hadn't worked, Buffy wouldn't have had to endure all the hardships she went through, Willow wouldn't have gone all Dark Magic-y, and Tara would still be alive. There must be such an alternate timeline out there somewhere, and Willow wants to go find it.
And surprisingly, the Faction agrees.
So, a few sacrifices and spells later, Willow finds herself in an alternate Sunnydale, sometime in an alternate Season 6. But what she finds isn't what she expected. With no Slayer around, Sunnydale has been overrun by one demon horde after another. The Willow of this timeline has had to use more and more magic to fend the demons off, and... well, nature has taken its course. She's gotten herself so full of Dark Magic that she's become bigger and badder than any of the big bads. She has essentially set herself up in the Mayor's old position. But this Evil Willow isn't the grief-crazed magic-addicted train wreck of the TV show. She's very much in control of herself. She thinks what she's doing is right, and if the Scooby Gang is opposing her (which they are), it's just because they don't understand the necessity of what she's doing.
(I have this one scene in mind, where Evil Willow has the three nerds working for her, and one of the ways she controls them is by teasing them with kisses and promises of making out later. Our Willow watches one of these episodes in disgust, and when the boys leave she asks her evil self, "They do know we're gay, right?" Her evil self purrs, "Are you kidding? That's their favorite part.")
Eventually, our Willow teams up with the Scooby Gang. They don't trust her at first, thinking that Evil Willow is trying to trick them, but Tara recognizes her right away. It doesn't take Tara long to divine what our Willow has been through, either, and it horrifies her. It causes some issues until Tara has a chance to take it all in. How do you folks think she'd react? I'm curious.
Anyway, on to the main plot - Evil Willow is able to probe our Willow's mind and find out about all the evil nasties from Season 7. She decides that if she doesn't botch the job the way the First Evil did, she can seize all the power for herself. The Scooby Gang has to figure out how to stop her, a plan that will require our Willow to use her full power. So much power that Tara (who has been growing as a witch through fighting Evil Willow) will have to stabilize her. The big confrontation and fight occur, and the Scoobies prevail - but there's a cost. Our Willow essentially burns out her magical abilities. Evil Willow is wounded and drained, but still has enough power to kill our Willow. But before she can deliver the coup de grace, Faction Paradox reappears. They had an ulterior motive all along - they want Evil Willow to join them. And Evil Willow agrees - why settle for one universe's magic when she can travel to as many alternate universes as she likes?
And so, in the end, Sunnydale is saved for now, but there are dangers to come. Willow uses her knowledge of her original timeline and all the things she's learned about magic to become the new Watcher of the Scooby Gang. She even contacts Giles in the UK to see about becoming official. She also starts working on a way to get Faith out of jail, since Sunnydale is going to need a new Slayer. And as for Tara... Willow discovers that she can still do small bits of magic when she and Tara work together, they way they did when they first met. Tara still has her worries, though. Willow has to tell her that things will be different. "I watched you die!" she tells Tara. "I had to go on without you! I don't ever want to go through that again."
Oh, and one more thing - at the end, JFK shows up at the magic shop, saying she's a potential Slayer who's been sent there for training. She eyes Willow, but before she can say anything, Tara wraps an arm around Willow and says, "Sorry, she's taken." Fade to black.