Rumors seem to indicate a Grey Knight codex.
Grey Knights don't use Neophytes in battle, because they haven't completed their psychic training and such. And when they have completed it, they're too bad@$$ to be mere scouts, and just go straight into full service.
There's plenty of room for expanding the Grey Knights, but we don't need copied and pasted units from the vanilla Marine codex. We're Grey Knights, not Grey Marines. We can do stuff no other SM Chapter can hope to accomplish, and there's plenty of room to add in new units that reflect that.
Like I said, lots of room for expansion. Tweak the Brother Captain into a chaplain/librarian style HQ for uniqueness, differentiate the teleport squad from troop squads more, add in jetbikes (rumored to be true, standard caveats apply), make up an elite(er) non-terminator unit or two and you've got a decent list of units right there. And that's just off the top of my head. GW's bound to come up with some crazy stuff to fill in the rest.
Edit: When I say Chaplain/Librarian style HQ, I mean a 2 wound model with special rules, not specifically a Chaplain/Librarian. Every Grey Knight has the faith of a Chaplain, and the psychic skills of a Librarian.