An Iranic-speaking tribal group, either near the Roman Empire and thus West of the Ukraine (like the Roxolani) or near the Sassanid Empire and thus East of the Ukraine (like the Alans) pulls an ethnogenesis and state-formation like that of the Franks, thereby absorbing various other groups, in which achieve dominance through some military innovation (stirrups?). They carve lands out of aforementioned empire or are alotted them as vassals / foederati / satraps / clients, plus they control the steppe where they came from, and much more of that steppe, extending into Ukraine. They acculturate faster than OTL Franks (more like OTL Noricans) to said civilization, and develop their own distinct variety, with a splendid capital like Atil of OTL´s Chasars. Their military strength allows them to defeat and absorb the equally Iranian-speaking Huns (and Avars, if their confederacy isn`t butterflied away).
They adapt irrigation technologies and transform parts of the Ukrainian steppe into arable land (it´s very fertile soil!) when population pressure becomes too high. Arriving Slavs are defeated and either enslaved or hired as labourers on the new agricultural fields; these Slavs adopt the Roxolan / Alan language over time, which in turn has absorbed some Greek influences itself, but not so much that it would have lost its linguistic affiliation to the Indo-Iranian language group.
This empire keeps other Slavic principalities as vassal states and practices shifting alliances with whatever is left of the Roman and Sasanian Empires, and later also with the states of the Muslims, if they still emerge.
Having grown in population numbers considerably, they fend the Göktürks and other Turkic confederacies off easily. Magyars and other Uralic tribes, like various Turkic tribes, are assimilating to this Pan-Pontic Empire.
The Mongols are best butterflied away; if not, well... hm, I guess after another millennium of Iranian cultural and linguistic dominance, the Ukraine would be left with an Iranian-speaking majority even if their empire is shattered by the Mongols and new polities emerge on its territory, even if these new polities have non-Iranian speaking upper classes.
