I really would've loved a "Ripper" spin-off. Giles is one of my favourite characters in BtVS, I think all of his backstories about living in England as a rebellious young adult and then it would cut back to his middle aged adult self -- sort of a "what the hell was I doing when I was younger" would've been so interesting, there could've been episodes with titles alluding to British-related cultural things, I was thinking "London Calling" as a potential episode title as Anthony and Giles are both from London, both would be from Camden, methinks!
I've never really cared that much for Faith, yeah sure it would've been interesting to find out more about her than we already know about her. I much prefer her in smaller doses -- as a recurring character that pops up every so often to be an opposite to Buffy, to represent what could've happened if Buffy had no positive influences or anyone who loved her or cared about her. Although, she did get a spinoff in the comics alongside Angel in the appropriately titled "Angel and Faith", as strange as this might've sounded, the Mayor might've "cared" about her but Angel really did care about her by trying to help her get out of her path into destruction. Caring in the eyes of the Mayor was more or less that he could buy her things like a PlayStation and a new flat, it was all monetary.