there's so much they can do with the CVII formula. and it was totally original. yea it was ok for SOTN to copy some things from Super Metroid to make it better, though NES really had nothing similiar to CVII. they totally revolutionized Castlevania in lots of innovative and more immersive ways.
I mean
Zelda II: The Adventure of Link was extremely similar.
And overall, just a better game. (though good god some of the later parts were absolute trash)
It's worth mentioning that innovation does not always make a good game. Castlevania II had a LOT of great ideas for its time, but it was too held back by the system's hardware to really deliver it, not to mention the game still played very similarly to the first game, despite being structured completely differently.
I don't totally agree with Egoraptor's Sequelitis on CV2 (particularly, his rants about "durr traveling is dumb") and all that, but he did hit a lot of points on the head with that game. It's open-ended-ness should have provided faster-paced gameplay to accompany the large world to explore. Faster movement, more fluid jump controls, faster attacking, etc., all of which Zelda II provided.
That's not to put Zelda II on a pedastal though. Good game for its time, but man, a lot of the later sections are garbage.