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.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.
Not really. Technically, all of Buffy's "demons" could just as easily be termed "aliens from another dimension," and its conception of magic could just be written off as being the same kind of advanced science used by the Carrionites in Doctor Who's "The Shakespeare Code."
The REAL problem comes from Doctor Who's assertion that it is virtually impossible to travel between dimensions now that the Time Lords are extinct, since interdimensional travel is so much more common in the Buffyverse and doesn't necessarily lead to universal calamity like Doctor Who claims it will if it happens too often. That's not impossible to overcome, of course...