I think Joss should pow wow with Russel Davies about a tie-in with Torchwood.
There's already a working relationship there (Head narrates "Dr Who Confidential among other things).
I figure the regular Torchwood folk are great at dealing with alien and technological threats, but are really out of their element when dealing with "occult" threats. My idea is they contact the Watchers for help during such a crisis, and Giles is assigned to them as a part-time consultant.
He'd guest star as needed, and this would let BBC test the waters, (and see how the character plays outside the Buffyverse) without first having to commit the money into tooling up for an entirely new series. Ideally, the character would prove popular enough that the Beeb would feel sufficiently reassured about the "Ripper" concept to give the character his own spin-off.
That would be cool, but from what I remember aliens don't exist in the Buffyverse and magic (at least how it exists in the Buffyverse) doesn't exist in the Whoverse.