4: Stumbling Into History (with aliens)
This rather horrible word is apparently standard Who fandom terminology for "a story set in history but with aliens and the like instead of only historical events," i.e. every period adventure for the last thirty years, including every one from the revived series.
Since it's been six whole Doctors since a "straight" historical, these have picked up most of the features of those (historical mysteries and disasters, meeting both famous and unknown people of the era, dressing up) as well as regular Who stuff like arguing with monsters and running from laser beams.
The earliest example was The Chase, which explained the Mary Celeste mystery by having Daleks exterminate everybody on board. That set the standard for answering historical mysteries by saying "aliens did it" as seen in quite a few of these. Grab a historic mystery (there are sites cataloguing them) and run with it.
It also provides an easy reason for the travellers to leave the TARDIS and follow the adventure all the way through, rather than leaving before things get dangerous as they might choose to in a straight historical. Something weird is messing with time, and sorting that out is their job!
Side note: While counted as a type of Who story, it's really more a setting for Who stories unlike the more has-its-own-rules straight historical. So it can and does mix with the other types on this list...
Anyway, what else does it provide us with, other than a more reliable excuse to involve the characters?
A broader "what if?" canvas to mess around with.
A mystery that can be cleanly solved without all the complexities of real history and actual people.
New or classic monsters doing something episode-specific. Daleks pretending to be WWII weapons or Clockwork Robots disguised as French courtiers.
A chance to play around with mythology as well as history, real-life unexplained phenomena, and the genre fiction of the time as well. Dickens meeting "ghosts" for example, or the legend of a stone circle being knights turned to rock, or what the Loch Ness Monster really is.
Classic juxtapositions of history and SF like Daleks threatening Churchill or Cybermen fighting Cavaliers. This is the shot that goes in the trailer.
Example: The House That Screamed
Following a strange energy reading, the characters travel to Winchester in 1588, where sightings of ghosts suggest that something from beyond is at work... Insubstantial alien beings? Psychic projections? Paranormal echoes of the past - or future?