The Timeless Child... The Lone Cyberman... Dr Ruth...
Thus far, this series is a little turbulent, but at least some interesting threads are being woven. It remains to be seen where they will end up, and if they were worth the investment. We live in hope.
One way or another, any arcs are better than no arcs. Threads ladled with mystery have been an essential part of Doctor Who programming, ever since the word 'Bad Wolf' began cropping up way back in the Eccleston saga. It adds interest, plays a long game, and gives the hyper-vigilant Doctor Who fanbase something to sink their teeth into.
It became such an institution in fact, that several or more threads began to emerge simultaneous, especially when the plot-mad Steven Moffat had free rein over the story telling. That's why this list is fairly chunky, and some of them are more sneaky, memorable and climatic than others. Please keep in mind that the higher entries are based largely on the impact of the payoff, rather than the longevity of the thread.