Some ideas about new classes:
Bard: Class skills are songs, one could demoralize enemies by reducing their damage by 1 (song of demoralize), one could boost hit points (song of bravery)
Monk: Class skills are one kick skill, one fist skill and one skill to improve some stat, maybe damage reduction by 1 for given amount of turns, unarmed damage is 1d6 instead of 1d3 and it grows to 1d8 by level 3 and to 1d10 by level 6.
Druid: One class skill would be shapeshift, probably werebear with increased damage and hit points.
Something I need to take out of my chest regarding the Bard class:
Bard is that one class I don't recall ever playing with and also the one class making no sense to have as an option to be an adventurer. I mean, a guy who spends his entire life playing musical instruments and travelling that also happens to know how to fight beasts. I simply can't picture in my mind having a guy playing magical music during a battle to add bonuses or penalties to characters -
"Come here you little bastard! Feel the wrath of my flute!" - it feels more like a parody to me. I think such mechanics should be part of other classes, mainly the Mage class, but instead music just using plain spells to add bonuses and penalties. I believe that the bard class exists just because it became a tradition to have it in roguelikes without that much of a thought.
The bard class could make sense for party based games as a support unit though. They are travelers, so their general world knowledge should be high as well as having high experience dealing with people making them exceptional traders. Ok, have them also playing magical music during combats but the effect should only remain as long they are playing it.
If you really need more classes:
Necromancer - It is so fun to mess around with the dead.
Summoner - A guy who specializes in summoning all sorts of beings.
Mentalist / Enchanter - A guy who as a power to use his mind as a weapon, including causing pain and paralyzing his foes.
Soulreaper - A guy that earns a lot with death. Like having his life or mana recovered by each kill or something.
Just some thoughts. I just made up some of them so...