4. Time-Traveling Clara
Clara's got a wrist time travel device! How cool is that? Correct me if I'm wrong (and I know you will), but I think that makes her the only non-River companion to time travel solo. So, why would Clara be time-traveling solo? There are three Doctors, for crying out loud, presumably with three Tardises. Why would she need to go it alone? Did she ditch the Doctor to go get something herself? Does she maybe have to get a component of whatever MacGuffin will get them inside the Time War? But maybe what often happens in finales happened to her: maybe the Doctor tried to protect her, and ditched her because of it. In that case, we will all exhibit total lack of surprise when she turns up in a flash of shiny light, grinning and ready to punch him. And guesses on what that thing in front of her is? The floor is open, people!