There isn't very much I actively enjoy about Chris Chibnall's time as the Doctor Who showrunner. It's not that all of it was objectively bad; it all just felt very middle-of-the-road. I'm also yet to meet anyone who approves of the Timeless Child storyline, so maybe that was actually just overly ambitious writing that went terribly wrong. Jodie Whittaker's Thirteenth Doctor could have had a much better era with a different showrunner, but it at least made sense that Chibnall would cast her as the show's lead, given their previous collaboration on Broadchurch.
However, there was once a possibility that Whittaker wouldn't step into the TARDIS until a later date, if ever. Speaking recently on 100 Questions With Tom Simons, former Doctor Who star Peter Capaldi spoke about being the Twelfth Doctor and how his time on the show came to an end.
While discussing the matter of Steven Moffat's departure, as well as other members of the cast and crew with whom he'd been working, Capaldi confirmed that he'd had "some talks about the direction" moving forward and that what was discussed didn't appeal to him. So, we almost got at least one more season with Capaldi. What would that have meant for Doctor Who?
Peter Capaldi staying on as Doctor Who's star would have been a first for the modern era
Doctor Who's classic era was in production when the TV industry wasn't quite so heavily regulated. While there were certainly senior creative minds behind the scenes, the term "showrunner" hadn't yet been coined, and the responsibilities were divided up very differently. Since the 2005 revival, Doctor Who has adopted a more modern approach to its behind-the-scenes affairs. Russell T. Davies served as the first showrunner, then it was Steven Moffat, then Chris Chibnall, then back to Davies again. Currently, it's quite famously no one, as the show is on a break while it finds its feet again.
Each time Doctor Who has changed showrunner, a new lead actor has also come with the transition. The opposite isn't true, though. Each serving Doctor has only been overseen by a single showrunner, with a few minor exceptions, like when David Tennant came back for the 50th anniversary while Moffat was in charge. Capaldi's recent comments about Twelve's exit sound like he made his decision based on a number of factors, one of which was that Moffat was leaving. If Capaldi had opted to stay on as Twelve, he'd have been the first modern Doctor whose era had been split between more than one showrunner.
How Capaldi staying on would have changed Doctor Who
If we assume that Chris Chibnall would still follow Moffat into the showrunner's chair, then the beginning of the era that saw Whittaker make her debut in 2018 would have instead been led by the grouchy, Scottish-accented Doctor rather than the first female version of the Time Lord. Therefore, Chibnall wouldn't have needed to undergo the soft reboot process that happens each time the Doctor regenerates. 2018's "The Woman Who Fell to Earth" was written around the fact that the audience was being given a new jumping-on point. Continuing Twelve's story would have made this unnecessary, and writing as though we'd never seen Capaldi's Doctor before would have been confusing.
That isn't to say there wouldn't be a marked change, though. Chibnall's style made his era feel immediately different. He changed the style in which Doctor Who was shot while he was in charge, and Segun Akinola was hired to replace Murray Gold as the series' composer. As such, the post-Moffat portion of Moffat's tenure would have looked and felt very different compared to his previous seasons. I think it would probably have been quite jarring, so I'm glad Capaldi left when he did.
Perhaps the most important question is what Doctor Who would have looked like further down the line. Whittaker left Broadchurch in 2017 and went straight onto Doctor Who that same year. If Capaldi had remained, that wouldn't have happened. Despite her history with Chibnall, it stands to reason that she would have accepted another job and been unavailable when Capaldi was ready to call it quits.
As such, I think it's reasonable to assume the Thirteenth Doctor could have been played by someone entirely different, which may have affected the need to bring in Disney for the 60th anniversary. Who knows? Maybe if Capaldi had done one more season, Doctor Who would still be flourishing under its own power. There's no way of finding out, not without a TARDIS and some reckless changes to the timeline.
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