WMG / Vice: Project Doom

If the series had continued, the ultimate Big Bad would have been Koshchei

Damian/Melaney seems pretty adamant in the end that BEDA has to be controlled by the exact same person in some way. Maybe not the same soul, but certainly the same genotype. So it effectively needs an immortal leader, but not in the sense of a demon. Given that both BEDA and Melaney are sourced from Russian, and the implication of demonic involvement in BEDA, the logical conclusion seems to be Russia's resident folkloric evil immortal, the warlock Koshchei. Cloning would just be a ready near-future supply of immortality if something happens to his body. Even better, there's a possibility that Sophia was intended to be a later Love Interest for Hart. Her family name of Koizumi doesn't have kanji supplied, but it can potentially mean "old fountain" (古泉), "blessed fountain" (幸泉), or "small fountain" (小泉). Meanwhile, Koshchei's target in The Death of Koschei the Deathless, Marya Morevna, has a name that can translate to "Maria, Daughter of the Sea"—and brine fountains are a thing. Hart could be pulling double duty as a tsarevich stand-in and threatened Koshchei life extension...