I'd say the big 'missing' core classes are
- a non-warrior divine caster (like the archivist from Heroes of Horror or the cloistered cleric from UA)
- a lightly-armored warrior type (swashbuckler, or the 'thug' fighter variant from UA)
- a warrior/arcande type (the duskblade and hexblade are spins on this)
And replacing the sorcerer with something more distinct from the wizard like the warlock or dragonfire adept makes a lot of sense. But then you've got 14 core classes, and that's a bit much; I'd banish the monk off to Oriental Adventures, probably move the paladin off to prestige class land, see if I can rework the fighter to cover the light-fighter role, and probably banish the druid off to a supplement as well.
- a non-warrior divine caster (like the archivist from Heroes of Horror or the cloistered cleric from UA)
- a lightly-armored warrior type (swashbuckler, or the 'thug' fighter variant from UA)
- a warrior/arcande type (the duskblade and hexblade are spins on this)
And replacing the sorcerer with something more distinct from the wizard like the warlock or dragonfire adept makes a lot of sense. But then you've got 14 core classes, and that's a bit much; I'd banish the monk off to Oriental Adventures, probably move the paladin off to prestige class land, see if I can rework the fighter to cover the light-fighter role, and probably banish the druid off to a supplement as well.