Sherlock and Loki as companions of the Doctor.

“The Doctor meets two clever kids. He is aware of how far their skills and witty minds can go, and he also knows what they will become. But for now, the children are the perfect companions.”

The TARDIS loves her boys, Loki is not amused by space, Sherlock keeps forgetting what Ten tells him about the planets, and the Doctor is going crazy, between deductions and a sassy little God of Mischief.