Happy thanksgiving, DMC GAF!
Everything about Eva show thus far (which admittedly isn't much) has just shown her as a sweet and motherly. I'd like to see whatever part of her made Sparda fall for her. I don't want her to be a damsel. Fearless and bold despite her own human weakness. Showing her demise at the hands of a demon, fighting to protect her sons. Like Morue said, that type of thing can be done really well and set up a "new" antagonist from the past that played a hand in her death.
Elaborating on Eva expands on Dante, Vergil, and Sparda by association. It just makes sense.
I know that you are not dismissing the idea completely but still I must ask, why not a love interest? I personally think that giving Dante a love interest and exploring those sides of his character could be really interesting.
A romantic interest could work, but Dante has this personification of being sort of a playboy bachelor. Not to say that something couldn't change him and it would be welcome, but he tends to just not take things too seriously. I'm just saying that it doesn't
have to be a love interest because that's sort of an easy way out, and the biggest issue for me is that if it's done poorly it could actually negatively impact Dante's character overall. The love angle is a natural easy one to force a character to care. There are a lot of other options. Lately, having a younger "daughter" type character has been a popular archetype (Walking Dead, Last of Us, etc)
Lady/Dante was the best executed pairing in the series, and it became largely platonic in the end. I think there's probably still some potential romantic interest between them, as friends that have some slight romantic attraction.