I'm a bit late to the party, but I just got Demon's Souls, and like many others before me pointed out, it's a very well designed game. While I don't agree that the game itself already resembles a Castlevania game, I can definitely see the potential of using a similar gameplay system for the series.
First of all, enemies are very challenging. Plus, there are this for the right reasons. You have to learn the patterns of their attacks before you can effectively dispose of them. You don't have acces to any combos, just a fast regular attack and a very powerful but slow one. It's like a little puzzle. Blindly attacking gets you killed very fast, so every action you take has thought put into it. Pressing the same button to initiate the same attack over and over again isn't going to work this time. Wouldn't that be a very good way to bring back challenge into the series without having to resort to making the enemies damage sponges? Combat like that is really satisfying, and when you finally figured it out you genuinely feel like you're a competent monster slayer.
Secondly, the game uses a lot of environmental hazards. You can just as easily be killed by a trap you didn't notice instead of enemies. I'm really suprised 3D Castlevania games have made use of them more. Is has a lot of simple but effective ones that would be perfect for the series since a lot of them are implented in a castle environment: boulders that come rolling down stairs, barrels with explosives that are triggered by enemies, enemy ambushes etc.
From what I've seen, this idea is very popular. I think it would be very refreshing change of pace from the recent combo based 3D Castlevania games. If you're going to steal, you might as well steal from the best, am I right?