I know Castlevania may not see the light a day again (that could be an unintentional pun), but I think it'd be cool to see Castlevania take the approach of a Souls game. Another one that I thought about a few of years ago as I went through them was if they bring back the Soul Reaver games, perhaps it could go in a Soulsborne direction. Soul Reaver doesn't utilize a lot of weaponry other than the Soul Reaver, but they could still do something I think. I think the world and style of both franchises can be expanded on with a Souls style game.
Obviously, it could turn out to be bad, but I'm going merely on assuming the final product is good, so take that into account rather than your faith in the end product when imagining what it could be if it ever existed. I've been recently playing Dark Souls 3. I'm currently trying (again) to play Lords of the Fallen, though on PS4 this time as it had originally been on the One as a Gold game but eventually became a free game on PS4. It seems to run better which is nice.