Let's say that some slavic pagan version of Plato rises in the mid 700s, and he creates a form of pagan renaissance in eastern europe
The kind of society that existed in mid-VIIth century more or less prevents this to happen amongst Eastern Slavs. Conquering religions are relatively rare IOTL, an handful, and are mostly associated (at least regarding their appearance) with roughly unified polities, and state structures to really support and patron religions.
Russia at this time is not what we'd expect from this, you'd have to wait the IXth century to really witness something roughly looking as an early state in northern Russia. : if anything, the more sophisticated ensemble in the region was represented by the semi-nomadic confederations in the south, generally Turko-Slavs, such as Bulgars or Khazars who never really managed (or even attepted) to undergo a religious policy even when part of the elite converted to an organized religion (Islam and Judaism in the case of Khazars).
As
@ArchimedesCircle said, furthermore, any Greek in 720's is bound to be Christian and is unlikely to reason in terms of platonicism, neo-platonicism or quasi-equivalents. Byzantium was quite aware of what existed in southern Russia,
and any Greek in Crimea would act in the already existing frames of relations and quasi-alliance with southern Russia's confederations. If anything it could lead to Christianized Khazars, ending up as a slavicized entity (altough less so than Danubian Bulgars). If you want a Rus Paganism, you'd need to work on inner dynamics.
Now, I don't think that it would be really workable to have a
triumphant Rus' paganism, even with this PoD. If you allow me, I gave my point of view why
there, but roughly it comes down to the idea that Mediterranean and Western Europe, being the core part of the continent in the early-middle-ages, was bound to have a cultural and structural dominance over the rest,
in particular when it met inner dynamics of reinforcement of power.
It doesn't mean that Rus' was bound to be Christianized (there's possibles alternatives about an Islamized ensemble as well), and you could see northern-eastern Slavic entities pulling a Lithuania given favourable conditions (altough less likely, would it be only due to the fact litterally sitting on the main trade road to Baltic and Black Sea) : but that would mean a significantly different Rus' with a possible even more confederal look-out (if you still have an unification of Eastern Slavic entities ITTL, which isn't obvious), which could mean a delayed Christianization undergoing Rus' conversion one sub-entity at one time, possibly
reinforcing the defense of traditional beliefs in some of them tough, until the XIVth (altough you may want to butterfly away Mongols).
Anyway, I agree with
@Turtle that a large revival of Rus' paganism in later history, Shinto-style, might be your best choice (especially if you had a Lithuania-like Rus'): a more lately converted Russia, with a strong nationalist feeling could pull what Japan did, and what is partially looking as such in XXIth Armenia, as in a nationalist-fueled unification and standardisations of beliefs, practices and rituals mixed up with "outer" beliefs.