"This isn't a huge surprise for anyone, but Mark Gatiss confirmed that Sherlock and Doctor Who will remain separate universes. He said that they "are never going to meet.""
Shouldn't be a problem at all really. Consider the Tardis has been to E-space, N-space, The space "in between" universes, The space so far away it was another universe, Alternate dimensions, and so on. This is the Christmas Special we deserve and it would be a ratings BLOCKBUSTER.
Update: If they cant devote an entire hour, then make it a one-off "Children in Need" special for eight minutes. I'd still watch the hell out of it.