If Momo's strength as a subordinate demon is influenced by the power level of her Mistress of Darkness Shamiko, wouldn't the natural solution be for Momo to transfer most of her power to Shamiko via blood consumption and then become a subordinate? In theory that way the much stronger Shamiko would ensure a hefty power support to Momo.
That's what I see happening in the future, at least. Maybe there's an evil magical girl they'll team up to defeat and Shamiko will steal her power instead.
I also like the idea of Shamiko consuming the luck curse from Milan as dark magical power to level up.