I wonder what percent of the universal visitors are British post HP. While Dr. who is popular there and pretty well followed in america, do outer countries follow it much?
I would imagine it is rather popular outside, but that Britain would be the predominant market for Doctor Who.
As for what they could do, I see a few ways they could do it. Logistically; some form of interactive element would be incorporated, hell; potentially with the Sonic Screwdrivers as a basis for the ride itself on that front.
And as for what they could do, perhaps looking at Justice League - Battle For Metropolis would be a good guess.
Take in mind, it's a thought; but I see it very likely instead of a expansion of Springfield.