I think any new boss concept is a good thing. This idea sounds solid to me. A succubus that had, until previously, seduced all vampire hunters that came to the castle but failed due to their own egos, weakness', etc, all woowed in by a vilumptous demon woman. Then the Belmont or whatever other hero comes along and is not tempted by her. So naturally she get's pissed and sics her hord of tempted sex toys on you which you subdue (kill or whatever) then after all that she takes you on herself. I think it would be a brutal because you aren't fighting monsters, your fighting humans which might have a more segnifigant sting on the Belmont's moral code of conduct. But the succubus herself is a demon so the hero won't think twicw about doing her in.
-X