Doctor Who has one of the longest histories seen for a TV show, spanning back to the '60s. In this time, The Doctor has faced hundreds of different alien species. When the show was re-launched in 2005, it was always exciting to see classic monsters return. These include the Daleks, Cybermen, Sontarans, and even the Zygons. But there are still many classic Who monsters that are yet to return.
This seems like a missed opportunity by the show runners, especially as they seem to rely on the Daleks and Cybermen a little bit too much.
While Doctor Who should always strive to introduce new monsters to excite the viewers, it can be entertaining to see old classic monsters return, especially when they're given a unique twist or interesting story arc to explore. Even the Autons, literal walking shop mannequins, were bought back and made quite intimidating.
And there are so many to choose from! With the correct script and right approach almost any of the classic monsters could return, but there are some where it is quite surprising that they haven't already returned in some capacity.