Was there any conceivable deal Germany & Russia could have made circa 1915 or 1916 ? Such a agreement would have to be persuasive to the Austro-Hungarian Empire as well. I've always assumed not, but want to run this one past any experts here for their thoughts.
I assume you don't want to change battle results, just diplomacy.
In April 1915, Russia has Austria on the ropes. Then in May/June 1915 we see a rapid reversal, but at no time is the core of Russia threatened. Between the fall of the Tsar and the Reds leaving the war, the Russians turned down a deal that was basically a cease fire in place. Things are no where near as bad in 1915 for the Russians, so this deal will not work. I guess the Germans could try a "return to prewar borders" but I can't see that having any realistic chance of being accepted by any of three powers.
In 1916, the Brusilov offense works then is reversed by major counter attack a few months later. All Entente powers try to hit CP at same time to win the war. In this mindset, hard to see a deal working. Any deal for same reasons as 1915.
Now if you want to start changing some battles, then I guess you could get a peace deal accepted. Or maybe have internal issues in Russia earlier. Or maybe a much different USA policy.