Mine would be a 2.5D Demon Castle Wars 3DS game, with 2D side-scrolling gameplay and 3D textures cel-shaded to emulate AK's art style. Of course, that means that Ayami Kojima would handle art, and Michiru Yamane would compose the soundtrack. Koji Igarashi would be the writer and producer, while KONAMI proper would be the developer.
With 2.5D graphics, players would be able to create their own character to fight in the Demon Castle Wars, and while the single-player campaign would offer a choice-driven progression and a multi-locale map system similar to OoE, optional multiplayer modes would include a co-op arcade mode, boss rush challenges, a shareable Castle Editor for user-generated stages, and a competitive arena.
As far as character creation goes, perhaps you would start by selecting a bloodline (Belnades, Danasty, Morris, or Renard), male or female, and some appearance options. Bloodlines would operate somewhat like RPG classes, with Belnades being stronger in magic, Danasty being more agile and stealthy, Morris being physically stronger and more versatile, and Renard being a hybrid of the three, for instance. Lecarde could also be a possibility, considering the events of PoR [and Bloodlines just before it]. Julius Belmont and Alucard would be critical NPCs, of course. Because of the potential variety of characters involved in the story, I would like to see some social elements / relationship tracking with the NPCs, something like Persona's Social Link system with branching dialogue and consequences, in order to add greater depth to story progression and skill levelling. This could potentially be tied back into a homesteading metagame like OoE's village system. I loved how OoE had a light Suikoden feel to it because rescuing villagers opened up new shops and dialogue in the game's town hub, and I think that could be expanded considerably in a new game.
Also like OoE, the game would feature Metroidvania-esque exploration but in a wide range of environments. Weapons and equipment changes would show up on the character models, and there would be several endings and difficulty settings to encourage subsequent playthroughs in a New Game Plus mode.
Even if KONAMI tries to ignore the Anthologies petition, such a game would still offer a compelling chance to include DS classics [and maybe even the GBA entries] as unlockables, along with common requests like a Sound Test and Art Gallery. Retail preorders should include a CD soundtrack.
What would you like to see as the ultimate next-gen Castlevania?