Recently reading the book "The German High Command at War: Hindenburg and Ludendorff conduct world war 1" by Robert Aspery, I came across the section dealing with the battle of Lodz and its aftermath, where German forces suffered 100,000 casualties when Ludendorff tried to take Lodz, the Russian supply hub, by frontal assault in December 1914. The author sites several contemporaries that complained about Hindenburg and Ludendorff being promoted too soon and were not ready for their level of command, as they missed a major opportunity for a local Cannae. The Russian offensive, they insist, was going to collapse due to supply troubles and the weather, not to mention the German fortress system on the border, which all would have left them open to a major riposte that would have netted Germany the Russian 2nd and 5th armies, with part of the 1st in said pocket. All without costing Germany nearly as many casualties and leaving Warsaw open to German forces.
http://en.wikipedia.org/wiki/Battle_of_Łódź_(1914)
So my what if is exactly this: Hoffmann suggests letting the Russians attack into the German fortresses, which would be backed up by Mackensen's 9th army, letting them batter themselves against German defenses before moving in on the depleted flanks and creating a mega-Tannenberg? Do you think this is even possible and what sort of aftermath are we looking at?
To add a bit of extra info, OberOst, German command in the East, was given the 4 newly formed reserve corps that were used at Ypres in December, which means that they would have had two armies to pull this off if they had waited...
My instincts say yes, this was very possible. In fact, it would be a major blow the Russians would have had a very hard time recovering from and it would force them to divert major forces from the Austrian front, doing far more to aid their beleaguered ally than what happened historically. The Tzarist army was known to be incredibly incompetent at just about any sort of organization, supply, or command issue at this point in the war, which gives the Germans a major advantage when trying to pull this off. Historically during the OTL version of the battle, the Russians had cut off a German corps and were preparing to ship 50,000 German prisoners away, when they broke out of the trap and took 16,000 prisoners, all their wounded, and captured enemy artillery with them! All because the Russian generals could not coordinate properly.
The battle would not be conducted until mid-December, but if properly pulled off, the Russians would lose their 2nd, 5th, and most of the 1st army. That is a major haul that would leave the Northwest Front eviscerated, prompting the entire Southwest Front to fall back taking the 4th and 9th armies out of the line to move them north behind the Vistula to over Warsaw and the new Vistula line. The Russian 3rd and 8th armies would have to then fall back, as they would be facing 4 Austrians armies (though probably beat up by this point), which might well cause them to abandon the siege of Przemysl, which in turn prevents Conrad from launching his heavily wasteful Carpathian campaign, saving something 800,000 Austrian troops and the garrison of Przemysl.
This has huge implications for 1915 and the course of the war, as the Austrians won't be nearly as worn down and the Russians will be much weaker when the Gorlice-Tarnow analogue happens. Italy might stay out of the war and Serbia could be invaded sooner, preventing the Salonika front from developing. It also frees up more German troops for action in the West, or at least means that they are more likely to be able to inflict more serious harm to the Russians, who now will be lacking irreplaceable officers, NCOs, equipment like artillery, rifles, and machine guns. Any replacement troops have far less material to work with later on. The 2nd Masurian Lakes battle might even be unnecessary, but if not, it would inflict still more irreplaceable losses on the Russian army.
However, I do not think it would end the war, just make it significantly easier for the Germans to win. It would also increase pressure on Falkenhayn to go with the East First strategy.