Idk, I just don't see it happening mainly because of the Hard drive. Even if they only slim it down by a fourth or fifth they still wouldn't be able to fit it. And I don't think it would work even if they made a compartment on the side to put the HD in. And I don't think MS would take away interchangable hard drive's. Also there's the massive power brick, which would look gay by a slim console.
Its might be possible to slim it down by a little but anything more is unprobable.







