And how would that be? Marcius was nothing, he lost fight with MC so not sure what was so important in eating him up in the first place. Knowing how it ended, my guess is she wanted MC to eat him so she can feed on him later and vampire's blood is more potent, just that I'm afraid. Not sure why MC was so engaged on doing that tho.
Whatever powers MC has still hidden, lore-wise there is no way he will be even near as powerful as Calisto. Maybe considering him a super gifted and talented one, he will be as strong as Calisto in half of time she needed to achieve that but it is still a lot of time and she was no match for even one of ancient so...
The worst thing in any story is being inconsistent about world. Dev can create world however he likes but even then, that world have rules that apply to it. If Calisto was nothing to weaker ancient, Virgil was nothing (and yet everyone was afraid of his power) then MC eating Marcius and being Calisto's child surely will not be strong enough to defeat both ancients. If dev will somehow will make it possible tho, it will be the same as George Lucas would make Anakin Skywalker turn into Super Saiyan while fighting Obi-Wan. He could right, but wouldn't make much sense? Serving ancient vampire and let her feed once a year is actually great ending to this story, there is no need to MC be a god of destruction.