If they do choose to go an open world route, I'd prefer the Dark Souls method over a sandbox.
Level design in Castlevania, at least to me, has always been a showcase of sorts. It's one of the major stars of the games, and in the best levels it has always been easy to recall "that spot" or "that room". Simon's Quest had this to an extent, but many of the "to and from" areas were flat and sameish. Portrait of Ruin was downright terrible about this. Portrait worlds were not only repeated, but the rooms within them were repetative. The non-castle portion of OoE suffered from this same lack of iconic design as well. Copy-paste four bridge, four exit, two tree room. Lots of flat hallways that tended to blend together. Most of LoS2's city suffered from this too, save for maybe the courtyard and the vista overlooking the massive chasm by the first Raisa fight.
Symphony of the Night, however, had this in spades. Aside from maybe a handful of hallways, every room was entirely unique in its design, and even the hallways had different challenges within them. OoE's castle channelled this spirit as well, and I enjoyed that LoS was able to keep the locales feeling unique as well. Dark Souls, while being open world, still has areas that have distinct entrances and exits, with unique locations throughout. Just in the first area, you have the dragon bridge, Havel's room, the boar gate, the barrel stairs, that tower with the obnoxious archer, the black knight tower, and more, and I'm sure anyone who has played Dark Souls will know all the locations I'm talking about just by reading this.
That's something I'd want to keep in Castlevania, and the logistics of making a huge sandbox world tend to take that away. Sure, you have a couple distinct rooms, like a courtyard, a park, a tower, etc, but then in between you have a whole bunch of samey roads, samey buildings, etc.