Thoughts on new base classes

With Heroes of Horror, Expanded Psionic Handbook, the Complete Series, Tome of Magic, Miniatures Handbook, Magic of Incarnum, and now Player's Handbook II, we have a ton of base classes available for characters, and it does not look like the amount of new base classes will get any lower. One may ask - do we have too many base classes now?

The answer I can give for myself is - no! Surprising as that may sound, I believe that there are still nieches untouched, or at least barely explored (transformative base classes would be one example). Also, new classes can easily occupy existing nieches as long as they don't overshadow the existing classes in that nieche (knight to fighter and paladin).

However, that doesn't mean that I would support the creation of n fighter-like classes that differ in their choice of bonus feats and class skills. A new base class should do one of the following:

• Provide an ability set that was not seen yet.
• Provide a mix of abilities that is hard or impossible by multiclassing between base classes.

Let's assume we want to build a cavalrist base class. We should first define what we want out of that class.

1. Obviously, he should be an expert in mounted combat. [That's a fighter's territory]
   
2. To make the mount matter, it should improve as the cavalrist increases in level. [druid, paladin or ranger]
3. He should not be a spellcaster.
   
4. He should be both suitable for the role of a noble knight fighting in mele and a barbarian scout fighting with the bow. [Fighter, paladin, or ranger (via a ranger's combat style)]

It looks like such a cavalrist might make a decent base class, tough it might end up looking similar to the fighter or ranger. That leaves one topic I haven't talked about - how specific should a base class be?
In d20 Modern, there are three types of classes - Base classes, which are broad; advanced classes, which are somewhat specialized, and prestige classes, which are highly specialized. Is that actually the case with Dungeons and Dragons as well? Not if you use the core rules as written, with the Fighter being a lot more flexible than the Ranger, the Bard more specialized than the Rogue, Blackguard being a prestige class and Paladin a base class.

Of course, the core rules are not necessarily a good measure, but I think there is just one answer - it depends on the game world. In some worlds, it might be necessary to have a Elven Leafgatherer base class. In the Forgotten Realms, why shouldn't there be a Red Wizard of Thay base class (in fact, this class would have the added benefit that it could be used as an arch-specialist class)?