Not sure where this one would lead. I can't recall seeing it discussed anywhere. Its been over three decades since I studied WWI in any depth.
The premise is luck and circumstance lead to the Russian armies wreck the 8th Army defending East Prussia, and about the same week as the historical battle ended the remnants are fleeing across the Vistula & the Russian cavalry are poking about the Thorn fortress. So, what happens next? I suspect this is a very wide question with lots of butterflies.