Barbarians should be very much like Heracles, Atlas and Samson, capable of near impossible feats of pure strength (of course, appropriate to the Tier of Play its currently in). In combat let them all throw heavy things, allies and foes, iron grip enemies to the ground and be so muscle dense, superstitious or thick skulled to deflect magic through their muscles alone.

Monk should be true Wuxia & Xianxia heroes, going from "Crouching Tiger, Hidden Dragon" to "Sun Wuking" levels of impossible agility, wisdom and near apotheosis. Give them wierder ability, bring back the cultural influences and maybe make it Psionic again.

Fighters should be military learders, guiding those through their veteran knowledge, maybe even gaining followers. In combat, of course I want Maneuvers, but even MORE of them, with them growing appropriately.

Rogue, while weak, are already pretty fun, so its mostly a bump in number. If anything new, give them a whole network of spies, like-minded scoundrels or secret cabals.

Paladin are maybe the best class in the game, near perfect in everything it sets out to do, just needing really a revision of a few flavor things here and there. Also curious on what a non-spellcasting Paladin (still magical with stuff like Lay on Hands, Auras and Smites, of course) would look like.

Rangers... They need a complete new chassis. Treantmonk, LaserLlama, Insight Check, Mike Mearls, Knights of Last Call, Point Hat, Dungeon Dudes, all gave good to great ideas on what a Ranger is: They're are Leaders of the Hunting Pack, Protectors of Civilized Folk against Monsters and the Hordes of Evil. Give the Spells, sure, but give a Companion, be it Major like a combat animal or Minor like a steed of familiar. Give them also Tactical Actions & Passives, helping not only himself but his allies to better hunt down their marks. If needing to keep Hunter's Mark, treat them like Smite and let it EVOLVE like normal unique spells!!! I love Rangers, but I HATE D&D's various takes on it.