Originally Posted by garbage456:
“Like the ice warriors. Sarah Jane. K9. Autons. Macra. Zygons. Master. Sontarons?”
Ice Warriors, Autons, Macra, Zygons, none of these were made as big reveals like Missy at the end, or like the Daleks at the end of Bad Wolf, or the Cybermen at the end of Rise of the Cybermen (and even then it's not a surprise, its in the damn episode name).
Very few of the classic characters are well known. The Master and the Daleks are really the only well known villains, with the Cybermen quite far behind in 3rd, Sontarans somewhere way off in 4th, whilst Sarah Jane and K9 are the most well known companions, and even they weren't some big reveal, they were shown at the beginning of their episodes.
If you brought back people like Leela, or Romana, or Ace, the majority of the modern viewership wouldn't know who they were, or perhaps vagely. Same with the Rani or the Meddling Monk or the Valeyard, the viewers aren't going to know them. Not saying you can't bring them back, of course you can, but you can't make it the big grand twist reveal like you could with the Master.