Unless IGA plans to put Shanoa in another game, or at least refer to her in some way in a new game, she's one of the most pointless characters in the history of Castlevania. It's always easier to put a Belmont in a game cause you don't have to explain that much - he/she is related to all the other Belmonts. End of story. But a character that doesn't even have a surname comes off as really anonymous in a series where it's always been about continuity (well, not
always, but sorta).
It would've been kinda easy for IGA to connect OoE with something - Like you said; AoS/DoS could've been an option. (Though it would've been even better if he had been
planning ahead with his goddamned storyline, and maybe mentioned Shanoa in AoS/DoS as some kind of OMFG LEGENDARY ANCESTOR

. Except he didn't know what games he'd be doing next, and therefor closed all sorts of opportunities for connection with characters from the past. If I was writing a long ass storyline, I'd be do A LOT of planning ahead, like, just come up with some random name and get back to it someday when I knew what to do with it. It's not that bloody hard, is it?) I know he comes up with the gameplay first, and then writes the story, but seriously... where there's a will, there's a way.