I think people are putting the classic games too much on a pedestal here. The series was indeed a side-scrolling action-platformer back in the 80s, but back then that hardly was an original idea either. Nobody is calling those games "Pac Land with a Castlevania skin". That's because they were building on a already established game concept, improved it, and added their own original touches. Obviously, they shouldn't just do a straight up clone game of Souls. People just want a game with a similar design philosophy. I see absolutely no reason to object to that idea since I don't think CV is a series that has its identity rooted in a specific gameplay system. It has always been more about the Gothic-horror atmosphere for me.
I get what you mean, but I wasn't only referring to the Classicvanias. Even as far up to 2008 with Order of Ecclesia and (to an extent) in 2013 with Mirror of Fate, Castlevania is still releasing the fast-paced action-platformer games.
On top of that, I never stated it should stay side-scrolling. Some examples I pointed out were 3D platformers like the Mario games or Yooka-Laylee. As well, I even stated that Ys Origin and Ys: The Oath in Felghana are closer to the style of CV than the Souls games are.
Platforming has always been a huge part of CV, outside a select few games. Again, I stated a Souls-style spin-off wouldn't be a bad thing, and I'd be all over that. But the main series should stay true to what it's about. Unless they somehow managed to meld platforming into the Souls formula... that'd be an interesting concept.
But that action adventure parkour element needs to stay -- I think "platforming" specifically hasn't aged too well and would feel clunky in such a combat-centric series, but updating it to a more Assassin's Creed/Infamous style of quick movement and navigation around large open settings with traversals, jumps, slides under obstacles, and diving from cover to cover in combat against a much more powerful foe could work wonders for updating the series to a more modern 3D style of play.
Freedom of movement in the style of Infamous would be great. I'd definitely be all for that.
Combat-wise, I think what would be a great example would be the Batman: Arkham series. The games have a very simplistic, yet fun and fast-paced combat system that I think could mesh well with a platformer if done right. Granted, with five Arkham games, Shadow of Mordor, and even Mad Max using this same combat system, I think it's probably best it's given a rest for a long while.