In my opinion, ever since Peter the great made Russia into a great power, it was and has always been impossible to conquer Russia and expect to hold much of it for more than a year or two. Simply because there is a lot of it, it's inhospitability, it's giant population (which was once a lot greater during the days of the sovjets) from which it's army can draw from, the fact that the people in it will hate any invader's guts and will do everything to resist them, and the fact that holding it's capital had historically proven to be little more than a symbolic victory. So in my opinion, anything Germany could have realistically done would do little more than change the date of it's defeat.