tabaqui recced this a few days ago and I'm seconding her rec. If you're in the mood for a nice long (17 chapters), completed AtS/BtVS ensemble fic go read
elgrey's
TempsPerdu (Time Lost). The focus is on Wesley as in this AU, the Senior Partners unleashed the apocalypse before Angel had a chance to assassinate the Order of the Black Thorn so to prevent the destruction of LA, Wesley threw himself into a demon dimension and Angel went after him. The Scoobies and the remaining AI gang unite to get them back. This actually happens early on, leaving the remaining chapters to tell of repercussions and some beautiful introspective analysing and tying up of loose ends of the past several years. I always thought that it was a pity that BtVS and AtS were so heavy on the action and that there was no time (partly due to legalities and logistics) for the 2 groups to get together and reminisce. The author truly gets the characterizations of Wesley and the others beautifully. Go read.