Well, I am opposed to class reductionism solely for its own sake, so before I give any other answers, I wish to register my grievances with the fundamental concept. "Less is more" is only valid when you actually DO MORE with less.
But, because I don't just want to threadcrap and have actually been thinking on it, sure, let's do this thing--and because I can use a thing I've thought on before.
See, I like the idea of the MTG color wheel. It's interesting, and can be exploited for both thematic and mechanical structure. I'm a sucker for places where rich thematic structure is, at the very same time, rigorous mechanical structure. And it turns out, we have a rather handy tool for differentiating stuff; 4e called it "power source."
My "six" (really, "five plus one") options would thus be
Martial
Divine
Primal
Arcane
Shadow
+
Psionic
Or, if you prefer "class"-style names rather than "source"-style names (not my preference, but there's no real difference in this context), that would be Warrior, Cleric, Shaman, Mage, Shadow,* and Mystic. Each of these then can specialize in offense, defense, control, or support; so a support-focused Cleric is a Priest, a support-focused Mage is a Bard, a support-focused Warrior is a Warlord (or whatever term you prefer). The Psionic source/Mystic class stands out from the others because it has no specialization--its special feature is that it picks and chooses from the whole scope of Psionic options, in a "the Green Lantern Ring is the most powerful weapon in the universe" kind of way. This would be balanced by (a) giving the Mystic very minimal baseline class features, so they have to leverage the synergy of their abilities to "keep up with the Joneses," and (b) including some kind of resource mechanic and/or reduced amount of abilities, to ensure that the greater freedom does not directly translate to immediately greater power.
The aforementioned specializations are something you choose early on, giving you a clear and defined facility with that aspect of play. All characters have the potential to grow into/"officially" acquire other specialties, or blend the benefits of two specialties in a way that supports both. All sources/classes except Psionic have an innate lean toward one of the specialties, and some options may require that you have a certain specialty in order to use them (or may get stronger because you have that specialty). E.g., a "Paladin" is a defense-specialized Divine character, with a variable side of healing mojo; by investing further into other support-related things, said Paladin may also pick up a minor or reduced form of support specialization, which when coupled with the innate support leaning of Divine characters, may allow him to be both an excellent protector of his team and an excellent provider of healing, restoration, and force-multiplication.
Loosely, each of the six associates with certain ability scores. Martial with Strength (or Dex), Divine with Wisdom (or Cha), Primal with Con (or Str), Arcane with Int (or Cha), Shadow with Dex (or Int), Psionic with Cha (or Wis). Doesn't mean other ability scores couldn't be relevant, just that those are loosely understood to be useful to most characters of the given source/class. E.g. a "Paladin" (defense-specialty Divine character) values Charisma, but also values Strength and Constitution because those are useful for wearing heavy armor, using heavy weapons, and taking hits. A "Ninja" (defense-specialty Shadow character) values Dexterity, but also values Wisdom and Constitution, because their fighting style is about observation, pinpoint strikes, and never getting hit in the first place (but being able to take it when a hit does land).
*Normally I'd prefer something else, but this power source is rather diverse, combining "dark wizard" types, ninjas, assassins, demonologists, necromancers, and alchemists under one roof. And most synonyms are either excessively bland, obscure (like "Larrikin"), or overly-specific (like "Spy" or "Ruffian").