So you've ended one series, possibly with a fleet of Dalek ships or the imminent destruction of reality as we know it or some other way of pulling out all the stops. Do you try and top it, or do something else?
12: Don't look back. Just go. RUN!
13: Is this it? Is this my death?
Well, one thing you may have is an arc. Time for it to pay off. If you can include a few surprise reference points as well as things the players have already guessed, so much the better. (We knew Saxon was involved with The Lazarus Experiment, but we didn't know why, or that the Doctor's handy spare hand was involved too.)
You can also pull a threat out of thin air (possibly literally) as long as it's been established elsewhere, and you do it in part one of two so the players have time to react. The Cybermen were revealed halfway through Army Of Ghosts, and then we got special guest Daleks in the last two minutes as well!
And on top of all this, you might have deaths, departures, regenerations, resurrections and more to deal with.
Or you might have a happy ending. Just to surprise people.
Example: Shadow Of The Stars / The Alignment
The Doctor finds the source of the Alignment - modern Earth. Someone is about to use the power of the Alignment to shatter the planet - and reshape reality as we know it to suit his needs.
Alien armadas (including the ones who kidnapped Richard's brother, and the invaders responsible for The World That Never Was) mass to blast the planet out of the sky before it can be used to reforge the universe.
A reunion with a lost brother. Time paradoxes running amok. Two imminent apocalypses. A mad plan that is, even just a little, tempting to join.
What, if anything, does the future hold for the travellers?