That sounds interesting. Maybe pagan remnants could survive in parts of Lithuania up to the Reformation
That's OTL. Large portions of the Lithuanian countryside remained pagan until the Counter-Reformation.
Hell, Lithuanian paganism even had myths about magical pistols gifted by the god Perkūnas, that's how long it managed to endure.
Let's say that an east-focused Lithuania conquers and unites the Russian principalities, and without generated strife and with many more East Slavic lands, steadily Ruthenizes from the lands it rules. Perhaps with the new center of the Grand Duchy of Lithuania becoming Kiev, Smolensk, or Minsk.
If this Slavicized Lithuania founded "Russia" as a Lithuanian and Ruthenian-dominated instead of a Muscovite-dominated state, how would it look like?
What could we predict in terms of economics, social structure, and urban centers? Perhaps it would look like a greatly enlarged Cossack Hetmanate?
Personally, I doubt that even with all of Rus' under Lithuanian control and Lithuanian nobility Ruthenizing, the capital of the state would stray very far from Vilnius. For the Grand Dukes of Lithuania, Lithuania Propria (ethnic Lithuanian territory centered around Vilnius) had always been the primary anchor of power, as it answered directly to the ruler rather than autonomous dukes or governors. It provided disproportionately more income and soldiers to the country than any Ruthenian territory (in the Battle of Grunwald, banners from Lithuania Propria composed half of the Lithuanian army, as an example).
To break this ethnic Lithuanian importance in ruling Lithuania, centralization even beyond what was done in OTL would be required, which first, would be even more difficult to pull off with so much more mass attached to it, and second, would take a long enough time that by the time the rest of Rus' start pulling their weight, Vilnius will already be established as a major population center with regional importance, so moving the capital out of it into Kiev, Polotsk or wherever would hardly be worth it.
It would not look like an enlarged Cossack Hetmanate for obvious reasons (wildly different story of formation), but I would not discount it evolving into something militaristic and absolutist, even possibly similarly despotic to OTL Tsarist Russia. Medieval Lithuania, before the Union of Krewo, was definitely not lacking in despotism despite its innate decentralization, and the old theory of this being an imported influence from annexed Rus states has generally been discredited in favor of it rising from the Lithuanians themselves.
Still, I also believe that Lithuania would be more Western-influenced than it's Muscovite counterpart. For one, it wouldn't have to fight for access to Baltic sea ports, as it immediately comes with easy access to them, so Western technology, culture and influence would flow in more easily.
In my opinion, an Orthodox Lithuania would look like a some form of hybrid between OTL early modern Lithuania and OTL Tsarist Russia.