The classes in the Player’s Handbook are specific enough in their mechanics and flavor that they can be modified and tweaked to create distinct classes that fit a similar archetype or class niche that might suit a character concept or a player’s or DM’s mechanical desires for a class better.
For example there is plenty of room in the game for a cleric type class that has something instead of turning or a tougher ranger type class without the beast master style companion and druid lite spells. A competent martial artist with different mystical powers or a fighter class that can encompass a knight or samurai’s court etiquette skills are all easy concepts that could be filled well by tweaking the core classes.
Wizards of the Coast produced spell-less rangers and paladins in the Complete Warrior and in Unearthed Arcana provided variations for each of the core classes and spontaneous casting for clerics and druids as options. Many third party publishers have produced their own variations such as the multitude of paladin classes found in Maladins Gate’s Forgotten Heroes: Paladins, the Unorthodox X series of class books from Thele games, or the slew of alternative ranger classes that can be found in any number of products or off the web.
Phillip J. Reed of Ronin Arts created a set of his own variations in Core Class Variants producing one variant class for each of the eleven classes in the Player’s Handbook, taking the Unearthed Arcana model as his template.
The eleven concepts are as follows:
Barbarian magic item destroyer.
Bard mystic musician
Cleric temple defender (½ casting, but fighter feats and a huge constant group buff)
Druid seed keeper benefits from a specific plant
Fighter Royal Guard
Monk ghost possessed
Paladin holy protector with stricter code and sanctions
Ranger Stalker bounty hunter
Rogue Urban thief
Sorcerer, self mutilation powered powers
Wizard battle mage (½ caster but no medium armor spell failure)
Unfortunately almost all of the presented classes in Reed’s work have problems on a mechanical balance basis. For instance the fighter variant loses their first level bonus feat but double their skill points, gain useful guard skills added to their list. On top of that they gain a power at first level to automatically prevent all sneak attacks in a level dependent range around them. Trading the feat for the skill points and skill options is an interesting choice that can be attractive for a straight classed fighter. However the ranged defense ability on top of the skills makes the variant class just mechanically superior to a normal fighter and makes playing a rogue that much more frustrating.
On the other end of the balance equation for a PC is the mystic bard variant who focuses on bard magic. They learn spells as a wizard and turn their instrument into a familiar type thing that benefits their magic while keeping bardic music, lore, skills, and their saves. In return they get reduced to wizard hit points and BAB, and weapon and armor options. That would seem an interesting balance as a wizard with lore and only up to bard level six spells sounds about right. However their main point (better bard magic) is severely hampered by the further detriments the class suffers in that the casting time for their spells is increased to a minimum of five rounds and they can’t cast any spells or use their music without their signature instrument. In combat this class is better than a commoner only in its bardic music.
The one that does seem most balanced and interesting for a PC would be the Stalker ranger variant that gives up animal companions and favored enemies for an increase in HD, skills, and some good urban bounty hunting abilities.
The monk variant also is interesting in concept and flavor but seems only suitable for NPC use as they give up some HD size, bonus feats, and are significantly vulnerable to turning in return for some minor fear powers and charisma damaging touch at higher levels.
Overall some interesting ideas and individual mechanics are in the Core Class Variants but the mechanical problems of the classes as a whole means that these need work to get them balanced to the point where they would be appropriate for use in most campaigns.