3. Go Meta
As evidenced above, Doctor Who has set a precedent that there are parallel universes. What if David Tennant and Billie Piper are not playing the Tenth Doctor and Rose? What if they are playing David Tennant and Billie Piper playing the Tenth Doctor and Rose? A timey wimey convergence of energy creates a wormhole that brings the two actors from our world to the Doctor Who universe and they must team up with Eleven and Clara to get them back to where they belong. Or perhaps the Cybermen, Daleks and Zygons team up to defeat The Doctor and attempt to pull a past Doctor from the timeline to kill. But their equipment malfunctions and they end up bringing the two actors over and mistake them for the real characters. It would make for a crazy episode filled with comedic shenanigans and tons of confusion for the characters and the fans.
Likelihood of Happening: 0/10 - This is basically the premise of the Season 6 episode of Supernatural entitled The French Mistake where Sam and Dean get transported to our world where they are actors named Jarod Padalecki and Jensen Ackles. It is a very fun episode that works well within the framework of that show. I highly suggest it. But I don't see it ever working for Doctor Who. Parallel universes? Yes. Our universe? No.