Here's the thing: Russia didn't just start to conquer stuff for the heck of it, there is an underlying geopolitical imperative behind it. Basically Russia is really, really, really easy to conquer beacause it is flat land and in the summer, anyone from Germany to Poland can reach Moscow quite easily. The Polish-Lithuanian magnates and their private armies had an easy time conquering the thing in the 16th cent. This is why any Russian leader with half a brain will try to push Russia's borders as far west as humanly possible, or as far west as he thinks he can get away with. Ideally, Russian power should be anchored in the Carpatian mountains.
So in order to have Russia end at Dnieper you basically have 2 options: (1) Russia only or mostly gets terrible leaders who don't realise what its national interest is - the result is that someone eventually conquers or at least breaks up the thing (Poles? Swedes? Turkic people from Central Asia? Russia's dangerous to all of them); (2) Russia is constrained by circumstances and can't expand, even though it tries - this essentially means they try but there is someone more powerful who stops them. Again and again and again and again. Because Russia will fight for dear life and try to expand again and again. Because not expanding means eventually there will be another Time of Troubles or another Tatar Yoke or even utter destruction. They need buffer states, whether those conquered peoples like being speed bumps for potential invaders or not (and I can tell you that we don't like it which is why it is in the self-interest of everyone around Russia to see it destroyed; Russia knows this so has even more incentive to neutralise all these threats).
Basically you either have the Russian Empire, or you have something very similar to Piłsudski's "Prometheism" plan succeed. In any other situation you're only on the road to one of those 2 outcomes.
I think for this to work you need someone very strong in the in-between region (between Germany and Russia). Perhaps a very strong Poland, or a (much) greater Hungary. Or an early unified and greater Romania or a surviving Bulgarian Empire stretching into today's Romania. Or a state out of Kiev. What i would do, personally is choose an early POD and play with the demographics - make all Romanians being Hungarianised early on, or better yet, Ukrainianised. Or all Ukrainians Polonised, etc. In order to have a stronger nation in the "Intermarium" region.
