Letting Castlevania experiment with different developing teams is certainly an appealing idea, but it's not a truly viable one in the long run. It's always nice to see fresh air breathed into a long-standing series; you certainly can't expect a genre to survive without a steady current of new ideas, and I've got nothing against hiring new faces to drive the old boat. But there's a point after which a franchise will need a certain sense of direction, and certain amount of consistency, to move forward. In other words, there's a point where you can no longer expect everything you throw at a franchise to stick -- and where letting producers do "whatever they want with it" might lead it even farther astray. Someone will be needed to preserve the overall form and coherence of the series, lest it become shapeless and empty like so many others already have.
Crisis has put it quite aptly:
Does the series need another IGA? I say yes.. mainly for the fact that we need another producer that shared Koji's distinct passion for the series, most notably the classic titles. An individual that respects the past by including the elements that made it great, while at the same time experimenting with new gameplay ideas that don't interfere with the overall experience. Someone that understands the importance of Castlevania's music and aesthetic.
Still, I'm a bit more skeptic regarding "separate 2D/3D teams" matter. If we're looking for that perfect blend of classic standards and modern innovation, then cutting the franchise in half won't exactly do it much good. We need someone who cares enough about Castlevania to bring both sides together, to cross an uncomfortable divide -- so we certainly don't need for that separation to become permanent.
While it's true that the current state of affairs is pointing towards that solution as most feasible, I don't think it is one that should be adopted for too long a time.
What we need is a single writer a la Amy Henning in the LoK series to keep the vision coherent, but various teams and producers.
Legacy of Kain. Such a great series, but look at what's happening to it now.
http://www.computerandvideogames.com/424304/blog/from-the-grave-video-of-axed-legacy-of-kain-surfaces/All that's left of it today is that ridiculous F2P "Nosgoth" spin-off that's in development for PC. Why, Square Enix? Is there truly no hope left for the series? Must it go down the F2P MMO road?
Anyways, your "single writer" idea isn't a bad one at all. Though it all depends on how deeply he'll be able to influence the series.
PS: Nagumo, if you're also asking whether IGA should return, then my answer is a definite yes.