Who would you have liked to have seen as a companion from any series of New Who?
Jackson Lake. I would have loved to see the Doctor and Jackson knock about the universe for a couple of episodes. And I wish RTD had written the Jackson Lake novel he mentioned once upon a time.
Lady Christina. I really think she needed to be in "The Waters of Mars."
And, like everyone else, Victorian Clara. I'd also take Dalek-Oswin.
For me Reinette from "The Girl In The Fireplace."
I think she would have made for an amazing companion to the Doctor.
I read a fanfic story a
long time ago that did that, and then put her back at the end. It was quite moving, actually.
The funny thing about this is that, just a few days ago, I was thinking about possible companions for the eleventh Doctor in case Matt Smith wanted to do Big Finish, and I was thinking that a story where the eleventh Doctor picked up Reinette for a bunch of adventures could be a lot of fun. (I'm assuming that, for BF purposes, Karen, Arthur, and Jenna would either be too busy and/or out of their price range.)