This is based around the "power drain" ability. MC is daydreaming as usual; he's a boring person in a boring world, nothing special, just average, but with great dreams and aspirations. He wants to be one of those amazing people.
One day, a disgruntled teenage girl literally pops into his life, his room in fact. She's actually a hypothetical construct, that is, a goddess born out of people's beliefs, and the strength of her power sustained so long as she's believed in. You can make up her back story and origin.
And as for the world of imagination, aliens and the Flying Spaghetti Monster and even Elvis can exist, but the waters can be murky here, so tread carefully.
But, this girl is here in his room because she's being punished by the Court of Deities and Supernatural Beings for causing a major disruption in their world. Her punishment is to be stuck to this average boy, of no special talent or skill, and lend him HER power for a period of however long the boy chooses to keep her around.
But, the catch is she can't let him mess with the world too much.
So, the boy now has the ability to steal the ability, talent, skill, what-have-yous of any person in the world, direct contact or not, whether it be a famous musician's playing or the rhetorical skills of a politician or the physical qualities of certain people. The transfer isn't permanent, but it can last as long as a year (cheap-o continuous renewal punctuated by time equal to theft, i.e. steal for a year, can't take that ability again for a year), or until the original ability holder dies. The amount of abilities he can take in is unlimited, but too much will put a strain on his body. During the duration of the power theft, the original person will be sans the taken ability.
That's the basic premise, now go!
The Virtues have seven days in which to track down their sinful counterparts and vanquish or otherwise incapacitate them. (The precise methods are up to you.)
The thing is, the Virtues don't know that they're Virtues until they get clued in by divine vision/prophetic dream/angelic visitor, etc. They live their lives normally up to that point.
Fortunately, the corresponding Sins and Virtues are alway close geographically, by a quirk of 'Fate'. (The idea being how fundamentally connected they are to each other.) So Envy might go to the same high school as Kindness, or Greed might work in the same building as Charity, etc.
Those sins that don't get vanquished are left free to leave their mark on humanity for the next seven generations.
You could have all sorts of fun deciding how to embody Gluttony, Wrath, Lust, etc. in the form of a person. What sorts of abilities they would have, and whatnot.
This'll get you started: http://en.wikipedia.org/wiki/Seven_deadly_sins
Lust Chastity
Gluttony Moderation
Greed Charity
Sloth Dutiful
Wrath Justice
Envy Hope
Pride Humility
Everyone on earth has a Guardian Angel. The angels are given to their human charges at birth. A guardian angel falls in love with his charge- although it can never be, of course, because humans can't see angels. Soon enough, he gets word from the powers-that-be that his charge is meant to die in an accident- but he can't bring himself to let her die, so he saves her... and betrays the hierarchy of angels in the process. Now pretty much all the angels try to stop him from saving her over and over again.
Tuesdays child is full of grace, Wednesdays child is full of woe, Thursdays child has far to go, Fridays child is loving and giving, Saturdays child works hard for his living, And the child that is born on the Sabbath day Is bonny and blithe, and good and gay.
Their interactions, foreign policy, discovery and culture shock, and ultimately, war or peace?
However, after X amount of time, there is a mass breakout at the prison and the assassin ventures towards Earth in search of the scientist he's come to befriend (and maybe more.) And anything could happen from there. ^_^
In what I wrote of it, there was alternating POVs each chapter, from the scientist to the assassin, and the story was largely explained by letters/emails. But it's open to interpretation, so tweak what you will. ^_^