There's been one banging around in my head a bit that I've actually wanted to chat with you about -- Person of Interest x BtVS, with Finch finding the SSN that came up is a very carefully crafted forgery, but definitely a forgery, so who knows what this "Faith" person has been up to. And then maybe keep going with SSNs of various other cross-over characters coming up on the Machine.

(You're more than welcome to run with this if you suddenly got an idea yourself.)