Worst thing that could happen is if the 3DS Castlevania is in 3D. I mean, 3D like LoI and CoD. The 3DS is certainly capable of pulling it off, but I don't think IGA is. Also, I strongly dislike playing 3D games on handhelds. It just doesn't work for me.
Having said that, I actually hope the 3DS debut is a Metroidvania rather than a haphazard 3D action game. There are many things they can do with the formula that hasn't been touched upon yet. Such as getting rid of the leveling up. Instead, they could have the player extending his/her health meter by picking up items similarily to how you'd go about it in the Metroid games. There's absolutely no need for leveling up in games like this. Also, your stats should increase depending on what armor/weapon you equip only. NO POWER LEVELING! I think games like Super Metroid have the perfect difficult level. The Metroidvanias could've been just like that, but instead they're all cakewalks - yes, CotM and OoE included.