Think Dawn's 'Key' ness could come into play (Dawn participation is part of the fic request). The First using her in some way somewhat parallel to the Scarrans wanting wormholes to go to earth to harvest birds of Paradise. This could be what brings Buffy/Spike to a common goal 20 years into the future (actually time-wise I'm thinking more along the lines of 15-17 years post "Chosen"). The fic challenge listed Dawn as an element, and I'm needing a way to work her in. Making her a weapon of mass destruction could be useful.