If you prefer two companions in the TARDIS, you may be in luck.
At the opening of Doctor Who's 50th Anniverary episode, The Day of the Doctor, we discovered that Clara was teaching at the very school that first companions, Ian and Barbara, did back in 1963.
In that first scene, Clara speaks briefly with another teacher, Tom Wald. At the time, we assumed he was simply a throwaway character, but now, it seems like there may be more to his story.
The actor who played Tom, Tristan Beint, was asked by one follower if Tom could be making a return. He responded with a sly, "Are you sure Tom could ever compete with a Time Lord though ... ? Oho ... Keep watching ..."
Pretty suspicious and, considering Beint also trotted out River Song's word of choice, "spoilers," we're certain we haven't seen the last of Tom Wald.
And while we're speculating, wouldn't it be neat if there were two teachers in the TARDIS again? After all, Peter Capaldi is the same age First Doctor, William Hartnell, was when he took the role. And Capaldi did say that Doctor Who was "looking back."
(via Doctor Who TV, Blastr)