I would say expect somthing a bit more Red Bull-esque.
They were the quickest single decker car in 2009, and I expect most teams to go with the tight packaged rear with blown diffuser integrated neatly into that.
The question mark for me is wether Mercedes will go with a larger sharkfin, as it seems they are very loathe to do so for some reason. Possibly somthing to do with their fancy airbox concept of 2010.
I see big revisions being made to the front of the car. The high nose of 2010, while in some way a nod to Red Bull, did not appear to do what Mercedes wanted(high drag for no apparent benefit in lower speed corners, a W01 achilles heel).
So maybe we will see Mercedes lower the nose slightly and incorporate more pronounced ridges in the nose a la Red Bull.
We are then left with the moveable rear wing, which according to some, may leave some scope open of some innovations. I wouldnt be writing this if I knew what it looked like
Then you are left with the the unseen things like flexing wings, weight distribution and and KERS packaging.