krieger
Banned
I don't think it's fair to only look at potential divergences that could have happened post 1384, considering that some of the ideas in the original post go as far back as the 10th century in PoD's. Thus we do have PoD's that could go as far back as the interactions of Baltic tribes with the Kievan Rus. Furthermore while I do agree that the Grand Duchy of Lithuania would most likely not be a naval or colonial power, it wouldn't be because of Muscovy being a threat, considering that for Grand Duchy of Lithuania to become, or depending on your perspective remain a regional or great power in Eastern Europe, they would have to break Muscovy before it grows too large to be contained.
I also have to disagree with the idea that Lithuanian identity and culture would disappear, considering how, as you yourself mentioned, ethnic Lithuanians lived for firstly Ruthenianized, then Polonized nobility for hundreds of years and finally one cannot forget even attempts at Russification in the 19th century. And yet the Lithuanian culture and identity, be it ethnic Lithuanian or historical Lithuanian still survived into the 20th and 21st centuries, though one cannot ignore the effect the policies of Russification had on the Lithuanian National Revival. Furthermore, one cannot forget that this Grand Duchy of Lithuania would be a completely different one from ours with a PoD that's 1384 at the latest or before in order to not be connected with Kingdom of Poland, meaning that it might not follow the same trends. Furthermore one cannot forget that already in 16th century you had people documenting and promoting the usage of the Lithuanian language among the nobility and the populace. And I am not even touching how much would having Livonia and Eastern Prussia, which has to be assumed if GDL is a naval power, would affect the culture and language in Lithuania Proper and even surrounding Slavic territories.
Also finally, even if in this case we are talking fully Ruthenian Grand Duchy of Lithuania, I don't see how this somehow adds on to the point that GDL should be replaced with Poland as a potential great power. Though I do agree that Poland should be added to the list.
OP stated clearly - "no Jagiellonian dynasty", so I assumed that he meant immediate pre-union period (times between Mindaugas and Jogaila). If we do Rus-screw in Xth century it's most likely beneficient would not be bunch of disunited Lithuanian tribes, incapable then of even succesfully raiding anyone. Bolesław the Brave IOTL managed to sack Kiev so if anyone would benefit from Rus-screw, most likely it'd be him - he had also another advantage over pagan Lithuanians, which was having positive opinion by upper parts of Russian Church. But still GDL would need to maintain a large land army and it'd not have control of Danish straits or of Black Sea straits (which was required to be a serious naval competitor in GDL's geographical position).
I didn't say, that it would disappear for sure, I just stated that it is likely. Russification started very lately (in comparison to Germanization for example) and it was done in shitty way by Tsars, who even didn't bother to alphabetize whole population of their empire. And arguably, even this ill-made russification attempts helped ethnic Lithuanian identity to survive, as it broke Polish cultural domination in the area (in 1863 even Samogitian peasantry was supportive of January Uprising, aimed to restore independent Poland and sang "Boże coś Polskę" in their native language). As far as polonization is concerned, it wasn't totally succesful, because Poland ceased to have regional power status and to even exist before XIXth century, which was crucial to mass assimilation. If it thrived, no one would bother to leave the union. Lithuanians might even still speak their ethnic language (but why even IOTL majority of peasantry around Vilnius or Kaunas were Polish speaking), but they would consider themselves Poles and show no separatist desire. In southern part of Ducal Prussia both nobility and peasantry used Polish and they still considered themselves mostly German in age of nationalism. Eastern Prussia was Baltic very little around time, when independent Lithuania had it's opportunity to take it. It has significant German and Polish speaking population, which together formed a majority. Shift from Slavic to Baltic is very unlikely and Livonia, if anything gives more German (not native Baltic) influence to nobility. And nobility (perhaps with townsmen) is a vessel for culture. Peasants are mostly looking up to nobles.
Maybe I expressed my thought wrong, I meant that Poland had equal (if not superior) potential to become great power as GDL.