Most Castlevanias had some form of exploration in them. Even in the MSX version of the first one. Castlevania II had a semi-open world. Castlevania III had branching paths. NES Castlevania and Super Castlevania IV, which was more or less a remake of the first one on NES, were the only truly linear ones. That's only two games so far. Rondo of Blood (once again) had branching paths. Almost all of them have had some sort of exploration or another.
SOTN fully absorbed Metroid and make it it's own. Have you played Metorid and Super Metroid? They're good, but they're also a kind of a pain to play through. (Go ahead, try to "wall-jump-sequence-break" effectively. It's almost impossible without a lot of practice.) SOTN (along with numerous other metroidvanias) streamlined the formula for everyone.
I really don't like Metroid itself, btw. Sorry, just saying. But that in itself should say that I'm willing to look at things objectively. I like the formula in other games and will give Metroid and Gunpei Yokoi credit where credit is due, but I just couldn't get into the sci-fi aesthetic.
Now, if they want MOF to go linear like in the first LOS, that's fine, but it wouldn't make sense due to the fact that CVIII had branching paths using the abilities of different characters. If not Metroidvania, then at least some sort of differentiation would be nice. It wouldn't make sense to do so seeing as how Alucard is all but confirmed for both games. You can't have Trevor without Alucard.
Edit: Also, they did the multiple-character thing in the replay of Dawn of Sorrow. With quasi-metroidvania tactics. Come on, you have to admit, that was kind of awesome. Yes, I know Julius Belmont isn't Alucard, but he still made for a pretty good explorable character with Alucard's and Yoko's help. Even Hammer was going to take over Grant's role, but he got cut out at the last minute. Aw.
LOS 2: potentially metroidvania. Mirror of Fate: Castlevania III's branching paths. Almost guaranteed at this point.
And notice how it's becoming more and more like the original timeline? They said after LOS 2 and MOF that they were going to stop the series. I agree. The last thing we need is another hideous anime mess. Please, just let it end.
Castlevania is a good series, but people need to learn when to let go.
EDIT: By the way, I remember what Gamepro said back in 2001.
"Devil May Cry is what Castlevania 64 should have been."
So yeah, you're right, Sieghart. DMC1 did take most of it's Gothic horror motif from Castlevania.
I mourned that day, when I read that quote, as I'm sure you would have if you had read it as well. Taking cheap shots at the 64 games was hitting below the belt. Especially since Legacy of Darkness wasn't that bad. It actually tried to improve the first version of the game. (It was more or less an expansion pak(haha) and/or director's cut)