Some of the best stories in all of Doctor Who's revival era are the two-parters: stories which span across two episodes with a nail-biting cliffhanger in the middle. Two-parters allow stories plenty of room to breathe, giving them a chance to properly set up the plot, setting, and characters, as well as do those elements justice later on. There's enough time in a two-parter to really explore the story in a way that makes them feel grander and more fulfilling than regular episodes.
With that said, there are a number of single episodes of Doctor Who that would have really benefitted from being two-parters instead. Perhaps they were overstuffed and needed their events to be more spread out, or maybe their ideas merited more time for exploration, or perhaps they were rushed and needed extra time to improve the pacing. Either way, these 45-minute adventures would have been far stronger stories had they been extended into feature-length two-parters instead.
10. Sleep No More
Sleep No More is one of Doctor Who's most unique and polarising episodes. It's filmed entirely in a found-footage style, like one of those cheesy horror movies from last decade. But surprisingly it's actually executed quite well, and the found-footage isn't just a gimmick: it plays a key role in the episode. Essentially, villain-of-the-week Professor Gagan Rassmussen's evil plan is to spread the infection responsible for the Sandmen via glitches in a video feed. That video feed is the episode you're watching, and you are the target for the infection. Basically, Rasmussen fabricated a Doctor Who episode and then used that episode for his nefarious schemes.
It's an ingeniously meta concept and it's genuinely unnerving, but what makes this episode feel somewhat unfinished is that the Doctor and Clara fail to save the day at the end of the story. They don't stop Rasmussen, they don't figure out what was really going on, and they allow lots of victims -- including you, the viewer -- to be turned into Sandmen. This episode ends on a great cliffhanger, but there's no resolution: there really should have been a Part 2 where the Doctor and Clara figured out what was happening and resolved the plot. Maybe then this unique and ambitious episode would've been remembered as a standout story.