Here's a link to "Idea #2."
Since they've already established Sherlock Holmes to be a fictional literary character in the Whoniverse, it seems impossible that Doctor Who and Sherlock could ever do a crossover. It is possible, however, if they made Sherlock a show in the Whoniverse and created a sentient holodeck character incident like Star Trek: The Next Generation did with Moriarty. Then they could get uber meta and mention how that happened on Star Trek: The Next Generation. You're welcome.