Hmm...for me, DMC has always been about people struggling with their otherworldy heritage - Dante and Vergil's relationship with Sparda, Lucia's origins, Lady righting the wrongs of her father who became a demon and Nero's journey to accept his demonic side (we now know came from Vergil) in a society that shuns demons.
So to get to the point, while I certainly wouldn't mind playable human characters, I wouldn't want them to take away from the main characters of the series who I feel have always embodied that theme. Lady is a perfect example of that, she's a major supporting character and she has parallels with Dante but we don't spend more time with her than we need to since she isn't the main focus.
Apologies if that was a little ramble-y and or incoherent. It's past my bedtime.