Neither so cliche as a world-dominating wizard nor so boring as The Kobold Busters. While designing some material for this campaign, I thought of an idea as to why the PCs and NPCs involved were not of more powerful classes (in-game reason, that is). I came to the conclusion that the other classes simply don't exist.
Gods are aloof and distant, and want to stay that way. Healers and Divine Minds, being such rarities, are viewed as prophets of good or ill and are thought of as minuscule fragments of the gods' will. Deities simply don't interfere in the affairs of mortals often enough to justify clerics or favored souls. The spirits of nature are equally aloof, and choose to manifest themselves in physical form when they wish to be known, rather than manifesting their powers through druids, spirit shamans, adepts, or even rangers.
Arcane magic is also an incredible rarity. Dragons certainly aren't likely to share their secrets, and their non-dragon offspring do not inherently have the capacity to cast spells, so sorcerers are simply never born (two half-dragons mating produce a spellscale, in case anyone was thinking of playing one and needed a fluff reason for existing). Likewise, no one is brave enough to amass enough stolen arcane scrolls to study, for the last man who had the audacity to begin researching learned arcane magic was met with swift retribution when the dragons learned of what he was doing. Hence, being a wizard is the last thing anyone wants to do, even if they could.
The knowledge of the martial adepts is scattered and difficult to come by. What little knowledge of their arts that is known is consolidated and taught to aspiring warriors willing to learn (via the Martial Study/Stance feats), but by and large the martial adepts are little more than myths. This prevents the existence of crusaders, swordsages, and warblades, while still leaving a wide diversity of warriors (fighters, monks, swashbucklers, paladins, knights, both types of samurai, and yes even the warrior npc class). However, a few eager youngsters will occasionally try to follow in the footsteps of these mythical martial adepts as they search for the fabled Nine Swords.
Even rogues simply have no place in this world. Youngsters showing a roguish inclination only truly have two paths in life: either they are taken under the wing of a swashbuckler looking for an apprentice, or inducted into the fold of one of the ninja clans looking for those with such talents (again, both ninja base classes are allowed). A third path does exist for roguish youngsters, but it is not one typically to their liking: a swift execution for their crimes.
A rough campaign concept, but I think it should help everyone get a feel for what to expect.
