First off... Bring back Ayami, for main character designs and promotional artwork. Vanillaware for enemy designs and animation.
The pandering Dragons crown is known for needs to stop, Include succubus and others if you want fanservice (After all, she exists for that purpose and I know it sells) But you know who also buys games? GIRLS. Don't over do the fanservice, it drives away the female audience.
For music, try to use more unique sounds (Like a theremin for example) Make your soundtrack stand out, and of course add the sweet horror elements castlevania is known for (Use Harpsichords, Organs, Etc,) Am I the only one who associates dracula with a creepy pipe organ? I think not. And the most important part, make at least 30% to 40% of the music remakes of the old series.... slightly more underground gems. We don't need another Bloody tears or vampire killer remake...
Black mass, Chandeliers, Flashback, Sorrows Distortion, Forest of monsters, Dance of pales, The Gears go Awry, A Toccata into Blood Soaked Darkness... WHY THE FUCK DONT WE GET MORE REMAKES FOR THOSE?
? If the PS1 could handle a map as big as SOTN, The SNES could hold metroid, Imagine the size of the castle we could make with our current consoles/PCs! Metroidvania is not dead, Iga was just uninspired.
How about including randomization like some indie games are doing (Rogue Legacy, Binding of isaac, Don't starve, FTL) on a small scale? Randomize the drops of weapons you find first from equal level pools. Maybe every boss could have a few different sets of unique battle animations that the game chooses at random for unique battles. (On my run medusa could fight me hanging from a chandlier while on your run she uses more petrification attacks while she fights you on the ground)
No more handhelds, the series belongs on modern consoles. The most successful Castlevania titles were not appealing to children. Don't be afraid to keep the series on 2D like they used to be. More and more 2D games have been successful in recent years. (Most Indie games, Rayman origins, Amongst many others)2D is making a comeback, and we all know most Castlevanias sucked on 3D. You cant have medusa heads (or candles without making it weird) on a 3D environment. And some might say its unimportant, but like theANdROId said... Its part of it "soul"
Come up with a story that takes more than 5 minutes to come up with, Ever heard of Final Fantasy? Maybe you have. They have wonderful worlds and stories. You can make a detailed story and still make it about dracula and the belmonts, (Like LOS but less predictable, If you start out with a bland story you can add all the TWIZTS you want... it will look forced.)
The story can branch out a bit more from the main quest (Ecclesia was heading the right direction)
Be proud of the titles that came before you. (You can do more than name a random character "brauner" as an homage to the old series) Specially when it comes to music.
Everyone loves symphony of the night... Yet all alucard gets to be in some shitty multiplayers and as a side character in games revolving around some nobody who is suddenly Dracula... Why doesnt SOTN get a sweet remake or a sequel? Not only would fans love that, but the whole gaming community would. Shit would spread by word of mouth like the old SOTN or even better now that most gamers have internet connections. Use the nostalgia to your advantage! Iga was going to before LOS but it probably would have been shit.
And thats just the surface...