What do you think would be the best nation for me?
Well, you showed interest in Bengal, didn't you? Xin Ming (i.e. southern) China conforms to your usual play style best, though (populism, nationalism, organising insurgencies in ethnic Chinese territories outside of the dynasty's control, modernising military and so forth). The Second Malinese Empire might work as well. France, alas, is no longer what it once was, but it can always go back to its good old Drengin-like days if a strong leader winds up in charge...
Sviatoslav of Kiev wins the Battle of Arcadiopolis, defeating the Byzantines and eventually capturing Constantinople?
No, seeing as Svyatoslav wasn't born. However, what did occur as one of the more immediate effects of the PoD was partially inspired by his OTL exploits, and indeed transpired as you had said. What made the difference was that this world's Kiev-based state had gotten involved in the Balkans earlier than in OTL, and had much closer ties with Bulgaria from the start, allowing a more peaceful personal union that led to a fierce war with the Byzantines, which the latter won because of the intrigue.
So the question is, why was Kiev more involved in the Balkans?
EDIT:
Ukrainian power migration
Define "power migration".

But if you mean heavy involvement of the "Ukrainian" elite in the Balkans, then yes.
forced Serbia to either a) move eastwards and come into conflict with Byzantium, whom the ultimately conquer
Err, how is that supposed to happen exactly? Unless the "Ukrainians" conquer Hungary and THEN press into Serbia, but that is not really what happened.
Serbs still live where they have lived in the Middle Ages, plus in Greece; they simply unified the Balkans (as detailed earlier in the thread; incidentally, that's an event of the second half of the 19th century, so it's not really very connected to the PoD).
b) somehow ally and/or conquer neighbouring South Slavic peoples in order to drive the Ukrainians away leading to an ultimate Serb conquest of Byzantium.
That's kinda what did happen in the 19th century, though Holy Roman Imperial assistance was perhaps more crucial than Serbian military and diplomatic successes in the Bulgarian Principalities.