Cleric, with a huge amount of domains and specialties to achieve everything from a traveling warrior-friar to a cannibalistic maenad. I demand more domains at launch than sun, war, and storm. I'm so freaking tired of those three. Give me lies, Hell, fatherhood, fertility, forests, hatred, thieves, prophecy, etc.
Paladin, given some mechanical mix between domains and warlock patrons encompassing specific kinds of knightly or religious orders in which the paladin was trained. Make a paladin's access to divine power different than the cleric's, maybe something like the sorcerer and magic, except instead of willpower, it's more tied to vanquishing enemies. Nothing like solving the 5mwd like replenishing resources through besting/slaughtering opponents (which could be done in very chivalric ways, or through blood for the blood god, as the need calls for). And don't even think of bringing back alignment requirements.
Wizard, Sorcerer, Warlock: I think they're on the right track with these guys. I want different approaches to magic, and they're in the process of giving them to us. And despite any similarities, their differences are both awesome and class-defining. Study, innate, and pact magic using Vancian, AERU, and spell points each bring something exciting to the table and account for an insane range of caster concepts. However, I think certain spells have always been problematic, and instead of rehashing the too-much too-little, beef and nerf cycle, I would dedicate magical classes to them, namely conjuration (Summoner) and polymorphing (Druid).
Summoner would encapsulate calling entities from any number of different places, and through specialties could range from prophet-like figures calling angels from Heaven, alienists tearing breaches into the Far Realm and unleashing shoggoth or Xothians, to planeswalker-like characters making pacts with creatures across different worlds to come and fight (kind of a reverse warlock, where you are the patron) or even enslaving them. If a class can be geared to calling other creatures to fight, it could also be specialized to allow for permanent golems and companion creatures/mounts, I imagine. And again, I feel it needs to be its own class because conjuration can become suspect, overpowered (or else so nerfed it's a kind of boring or lackluster option when included in normal spell lists), and kind of a DM hassle very quickly. Dedicate a class to it, playtest it extensively, and I think we could see it really open up.
Druid, with a long list of specialties serving everything from spirit-calling shaman, skin-changer, to hexing witch doctor. Launch also has to account for lots of wild shape forms, not just the typical wolf and bear. Like with the Summoner, I feel shape-changing and polymorphing magic deserves a class to get right. I don't want something broken or nerfed in a regular spell list, I want a legitimate treatment. Plus I am firmly in the camp that there is a 'primal power source' that is tied to primal and ancestral spirits, decidedly different than the sorcerer's penchant for elemental magics (though there is some overlap).
I'm kind of on the fence whether we need a primal gish like paladin is for divine and sorcerer seems to be for arcane. I loved Wardens, but not sure if they can't be served through specialties.
Rogue (with Bard and Assassin specialties), Fighter (with Monk-like unarmed potential and specialties to serve more Wuxia interests if desired), and Ranger, because bread-and-butter martial options deserve as much consideration as magic. These are all flavorful options that encompass a huge swath of character concepts, all of which I find interesting.
Warlord, because I do believe in non-magical healing in conjunction with grit, tactics, leadership, and maybe some sort of late-level mass-combat incentives. Look, it exists, and there's a portion that wants it. Don't include them if you don't want to, I mean there's always the Leader specialty.
Psion closes out my 12 classes, as I do believe psionics have a place in fantasy. Not every fantasy, but they do exist in the genre and D&D. Telepathy, telekinetics, astral projection, force magic, mental constructs, crystals, metaphysics, mysticism, time travel, emotions, psychological Archetypes, perceptions and reality, sanity, Far Realm and aberrations, I think it all scrapes the surface of what psions could be.