I think it could be
Somniloquy by Gwyneth Rhys. On EF and
AO3.
Summary: What if Spike had actually saved Dawn, and thus saved Buffy, in The Gift? Does being a reluctant hero change your status with the people who are reluctant to admit your heroics? And would Buffy know it if the ending of her story was supposed to be different?
In the second part Spike offers Dawn a key with this note: «This doesn't open anything. It just is.» Dawn was worrying about her purpose in life after the battle with Glory. Spike told her that she would be appreciated for being just her and not the key.
Thought I'd drop a word if anyone's interested in this topic.