Thanks for all the comments guys.
Random unconnected launch night idea....
Opening Line: "You're watching Big Brother... but is Big Brother Watching You?"
Launch night then plays out as normal, but in addition during the show we'll be revealing one more housemate who won't know they're moving in until Davina tells them - NTV style.
e.g. John Smith is at home watching the launch show. Davina then reveals that in addition to tonight's housemates, a "Wildcard" will be joining the housemates too - and we're about to meet them - only they don't know it yet.
Davina: "I am now going to speak to the house - not this house, but it could be yours.
"John Smith - This is Davina. You're Live on Channel 4 - please do not swear"
Davina then explains that they've been chosen to be a Big Brother Wildcard and will have the chance to move into the Big Brother house tomorrow, and asks if they will accept the opportunity. They then have the rest of the show to pack and make their way to a waiting car.
The next night the Wildcard will move in live - but of course there is a catch. They are the Wildcard - not a housemate, and in order to become a housemate they will have to earn the status of Housemate over the first week. If they fail to do so by Friday they will be evicted.
Over the first week the Wildcard will complete a series of rather gruesome challenges, sometimes alone, sometimes having to get the help of others. Should they succeed they will never be told anything more than "You're one step closer to becoming an official housemate" - as whatever happens, next Friday the Wildcard will get the opportunity to become a housemate.
On Friday during the first show the Wildcard will be informed they are now eligible to become an official housemate - but then the twist is revealed. To gain housemate status they must steal it from another. The Wildcard must then choose one HM to compete against for Housemate status, with a live task determining the outcome.
The winner becomes a housemate, the loser is evicted. Although as technically they aren't a housemate, they can't be evicted - but they'll still be getting the boot!