I sorta agree with Psion.
Basically what I think could work would be a system that is, at its core, based around skill points and feats. You automatically get a bit of attack bonus, save bonus, and maybe AC bonus per level -- the so-called "Dude Factor," where even weaklings know a bit about being cool if they're high enough level.
Each level grants you one feat and, say, 8 skill points. You can build stuff free-form, sure, but there would be solid, clear examples provided: Classes.
Fighter -- Uses its feats for combat tricks. Uses its skill points for attack skills, defense skills, and various maneuvering skills. You'd want to trim down the number of skills available, and make them all pretty good, so that a fighter might be tempted to take, say, Acrobatics, instead of that extra rank in Grappling.
Rogue -- Uses its feats for trap-breaking, sneak-attacking, and Han Solo-esque tricks. Uses its skill points for a bevy of skills.
Wizard -- Uses feats to access different types of magic. Uses skill points to learn each type of magic. I'm basically going a very 'Mythic Earth'-style here, where you might take a feat for "Greyhawk Guardmage," which gives you the ability to spend skill points on Defend and Divination spells.
Cleric -- Like a wizard, but chooses different magical feats and focuses on fewer types of magic, but has more combat ability.
Barbarian -- Like a fighter, but doesn't waste feats on armor, and instead puts feats into reckless attack abilities.
Ranger -- A combination of rogue and wizard.
At the core of the system, it would be entirely free form, but there'd be 12 example paths that would form the iconic heroes of D&D. Barbarian, bard, cleric, druid, fighter, monk, paladin, ranger, rogue, sorcerer, swashbuckler, and wizard.
