The potential for reality-tampering is set up by the very silly "Superstar", which is one of my favorites from season 4 because it's so silly. I liked Jonathan, and was disappointed to see the character ruined as part of the Legion of Dim.
The reality-tampering that produced Dawn is, however, an act of assault on the mental integrity of the people affected. It's non-consensual and, in my book, extremely bad behavior. If it happened to me, and I figured it out, I'd freak. My sense of self would be attacked: you mean my sister is imaginary? You altered my brain? No, sorry, you don't get to do that.
And it was done in service of a fairly stupid scheme. Buffy was utterly unable to protect the key. And even more unable to protect it because she didn't have a clue what the issues were, what was at stake, until very late. Some of it was because she was unresourceful and in a long, slow descent toward suicide the entire year. But really, couldn't the monks have just told her what the deal was? (Yeah, yeah, no season 5 then. But I do hate plots that depend on one of the protagonists being idiotic.)
Also: Dawn is not real. Dawn is not a human being. Dawn is a construct, rather like the Buffybot.
Unless something other than God can create life and give something a soul. And I am not ready to cope with the changes to the Buffyverse cosmology that little idea would entail. And yes, I know the real answer is that the writers were fallible, even in writing one of Buffy's best seasons.
But I promised a story germ. What if Buffy, instead of being a puppet to the monks' every silly whim, reacts with anger and horror? And Giles disagrees? Or vice versa, but that's kinda close to the impasse they were in when she was such an idiot in the season closer.
[ Aside about that conversation: Sorry, moron, if it's a choice between sacrificing your sister and sacrificing the world, you have to kill your sister. Be reasonable. No one is saying it wouldn't suck... How did Buffy know that the tower dive she did would even work? And that idea has already made for a wrenching story, the amazing "All Set Down" by desoto_hia873, complete with Scooby idiocy afterward. ]
Though I might have a hard time getting the practical Giles, with his oath to protect this sorry world, to oppose Buffy on this. The situation needs strong conflict, though, from somebody with some moral authority.
Things to explore:
- how she figures it out
- what Giles does with the information
- how she reaches the decision to kill her own sister
- how the people around her react
- what happens after the death: do the memories remain or vanish?
- what it does to Buffy if they remain
A grim story in many variations, I think. Or I could write a purely SFnal approach to it, with resourceful protagonists figuring stuff out and solving the problem practically.