What could be some Slavic Great Powers that aren't either Russia or Poland?
It would probably help if one of the two countries conquers (or inherits through dynastic maneuvering) the other in addition to Constantinople, Greece, and some or all of Anatolia. That way, South Slavs would collectively outnumber the Greeks, Turks, Armenians, and any other ethnic groups in the resultant empire.As Mr_Fanboy said, Serbia and Bulgaria. If either can take Constantinople and hold it, that would certainly help boost Great Power status.
Of course, the Serbs or Bulgarians would have to maintain their Slavic culture and not assimilate into Roman-Hellenic culture, though.
Well, successor states could still become great powers. It's hard to imagine that kind of religion though. Islam arose from the codified religions of the Middle East and the Indian continent. It took several centuries of written religious thought for something like Mazdaism and Yahwism to emerge and then over half a millennium for a full fledged Zoroastrianism, Judaism and Christianity to finally come out, the three religions without which there most likely wouldn't be an Islam. Slavs, on the other hand, are rather isolated from written cultures, so they always kept their oral religion which was even more disparate than that of Germanics, which has a more identifiable and unified pantheon than the Slavs ever had.Left field option: the Early Slavs expand not as bands of disparate tribes, but as a single military force inspired by the fervor of a new religion inspired by a Slavic counterpart to Muhammad. Much of Western Eurasia is conquered by this pan-Slavic empire, though it will almost certainly collapse sooner or later.
Actually, short of the religion and succession issues and one lost battle the Grand Duchy had a very good chance to became a Slavic superstate. Witold was in possession of Belorussia, Ukraine, Smolensk area, was acknowledged as “protector” of Novgorod and .... surprise, surprise, of the Grand Princedom of Moscow. He was planning to support an overthrown Khan of the GH, Totkhamish, on a condition that after being restored the Khan would recognize him as an overlord and will pass to him a direct overlordship over the Russian princedoms. The schema failed after defeat at Vorskla but the battle could be won or Yesugei could be persuaded to change sides or could die before the battle, etc.Lithuania! Now you will object that Lithuania s a Baltic, not Slavic nation. I reply: that is true today, but the Grand Duchy of Lithuania in the 15th century had more East Slavs (what would today be called Ukrainians and Belarusians) than it did ethnic Lithuanians and most of its territory was in what is now Ukraine or Belarus (or parts of Russia that have been claimed by Belarusian nationalists):
Moreover, the formal name of the state was the "Grand Duchy of Lithuania, Rus and Samogitia." And the "Rus" language was the official language of the Grand Duchy--the Statutes of Lithuania https://en.wikipedia.org/wiki/Statutes_of_Lithuania for example being written in it--until Polonization led to it being superseded by Polish and Latin.
Of course for the Grand Duchy of Lithuania to survive, it has to resist being absorbed by either Poland or the Grand Duchy of Moscow. But if it somehow manages to do so, there could be two East Slavic states--to use their Latin names, we could call the western one Ruthenia and the eastern one Muscovy.
There are even alternative ways to get a Ruthenian or West Rus' or Ukrainian-Belorussian state created. For example as a Swedish vassal state if the Swedes had won the Great Northern War. Or there was Oginksi's project to get the Tsar to restore (a mostly Belarusian and Ukrainian) Grand Duchy of Lithuania, which I discuss at https://www.alternatehistory.com/forum/threads/wi-russia-reestablishes-lithuania.479966/#post-19907587