Eh... if I had the authority, I wouldn't bother even trying to bring back Sonia. While I'm sure that may appease her group of fans (maybe), gratifying them with that act is just not the best course of action for the series to take, in my opinion. "This is the character from the (very obscure) game who was retconned, but we're going to reinvent her, change her story, and put her in the main canon, because... I don't know!?"
I just feel the resources and energy to do that would be better spent on something else and it would also be a clumsy thing to do. Again, I think Shanoa is a really good character that any girl should be able to respect. I understand she's not a Belmont, but as some of us have been getting at, a female Belmont just doesn't work that smoothly. I don't want to seem close minded, but it needlessly presents some problems and makes things a bit clumsy.
Yes, it can be done, and yes we're talking about a fictitious universe, but what is she going to do? Pass the whip down to her nephew? It just doesn't have the same weight as passing it down from father to son, and it would make it seem like her own children are/would be slighted (which they would be). One would have to explain how that would work, and yes, it could be part of the story, but... eh... why? And if it's not explained, then some people (like us) would be left wondering how they worked all that out.
I don't have any problem with strong, warrior women lead characters (like Shanoa), but I do think that, while a female Belmont can be done, it presents some issues that complicate things and are the sort of issues we shouldn't even be thinking about.