In my mind, the dementors have become an enormous plague in the world. The Doctor didn't know they were on the planet because they were hanging out in Azkaban. Then he sees one on the street and begins to investigate, running into a task force led by Harry.

I like the contrast of a nefarious soul-sucking Dementor legion versus a whimsical Harry and Ten. I know that Nine is a darker Doctor, so that could work easily, depending on the approach you take.

 

Time Lords are more towards immortal, actually. Not invulnerable, not at all, but this is the standard canon for Doctor Who now. After thirteen lives they can be resurrected, and even given a new cycle of regenerations. Although, as Gallifrey stands at the moment in the newer series it may no longer be possible.

Eh.

 

Well that's appalling. I hope as he is the last of the Time Lords, the Doctor has no idea how to do this.