I recently waxed eloquent about some things I disagree with on the show, so I’d probably change most of that just out of personal preference, but that’s a bit ambiguous and, I think, would have minimal impact on the quality of the show.
Speaking purely from a storytelling perspective, I suppose I would just change pretty much everything about Capaldi’s tenure as the Doctor. Cast someone else as the Doctor, give him a strong, one-episode introduction like they did with Matt Smith, have fewer of those “meta” type episodes, avoid Bill Potts at all cost, etc. But that’s aiming a bit big and large, I think, for “changing one thing.”
I think the thing that annoyed me most about Capaldi’s tenure was how he was still the Doctor, still playing his part in the cosmic struggle, and yet was less interesting than the other characters, most of whom surpassed him and the need of him at some point. He was somehow a force to be reckoned with, and yet, simultaneously, the next best thing to useless. I’m only just now starting to wrap my head around that.
But, really… I mean, really… if I am really going to change one, single thing about Doctor Who, I suppose it’s going to be, quite simply:
Casting a redhead as the next Doctor!
I mean, Tennant and Smith both had opening lines about wanting to be ginger, so why not, eh? 🙂
A friend of mine was rather fond of the idea of casting Rupert Grint, but did you know that Benedict Cumberbatch is actually, naturally, a redhead? 😉