Ace

Dorothy "Ace" Gale (or Gale McShane) would've been a companion of the Seventh Doctor, had the Valeyard not defeated him and merged with him in the Matrix.

A Life that Might Have Been

Ace would've traveled with the Doctor, and later on Thomas Hector "Hex" Shofield would've joined them. (He Jests at Scars... (audio story),The Harvest (audio story))