In the latest episode (Escape To L.A), in the server room, "Hitman" was about to pop Gwen in the head, while explaining who hired him (people who Jack obviously had a strong connection to) and why.
Who do we know that would have that much of a grudge against Jack, that they would go out of their way to make everyone on Earth immortal, just so Jack would become mortal again, and hunt him down?
"Hitman" was saying something about Jack taking something from "Hitmans' " employers a long time ago. Jack didn't know what "Hitman" was talking about, which leads me to believe that he has either:
A. Forgotten over thousands of years
B. That his memories were forcibly removed.
If you remember, the Time Agency that Jack worked for stole 2 years of his memories. This was mentioned when Jack first met with the Doctor in "The Doctor Dances".
Perhaps, another Time Agent, someone very high up, became stranded on Earth in the 21st century, possibly the one who actually stole 2 years of Jacks life. But why would anyone steal 2 years of Jacks memories? Something to hide from Jack?
In either case, I think, the Time Agency will some how end up being involved with Miracle Day.
Just an idea. Lets hear yours.