Some thoughts:
Don't make a string of human nations and then throw the other races in... make each nation the domain of a different race, so you've got an ogre nation, a dwarf nation, a goblin nation, a human nation, etc.
Then take each of the races and assign it a few of the core classes. Using the extended core classes of the complete books you can spread out the different warrior/priest/mage/rogue classess. So you have dwarven paladins, hobgoblin fighters, human swashbucklers, bugbear samurai and ogre rangers. Try to break down a few - but not all of the stereotypes so maybe hogoblin paladins but maybe not ogre swashbucklers.
Of course some classes are going to be everywhere - like fighter, rogue and cleric, but restrict others to certain communities - maybe elves do magic like sorceres, dwarves like warmages, orcs like warlocks, and ogres like wu jen. By making these class-based restrictions you're not ruling out the exceptions - just establishing the 'arcane culture'. The same works for the 'divine culture' (druid vs. cleric deity 1 vs cleric deity 2 vs. favoured soul vs spirit shaman), the 'rogue culture' (pirates vs. ninjas vs. scouts vs. spell thieves vs. yakuza vs. whatever).
This can be used to establish some cultural references for each of the nations. But again I'd say don't go for the sterotypes - don't put the ninja with the samurai break them out so it's not just 'Oriental Bugbears' and 'European Dwarves'.
Also consider bringing in Monte Cook's core classes for different twists on some of the same things to add to your options.
Add whatever additional ingredients - such as psionics as you want
On making the disparate cultures if you want them to be geographically fairly close - maybe establish some reason that they could have developed so differently without any ‘bleeding over’ from cultures. If you consider the similarities between European cultures you can see how it’s all routed in the same background. Yes there are differences but nothing like the differences between the swashbuckling pirate and the ninja… or the amerind warrior and the european knight. I’d say consider having certain ‘barriers’ to cultural transference that have relatively recently been conquered… maybe the seas are particularly stormy and so movement from one continent to another was difficult even though they were relatively close (Europe-Africa), but the seas have become calmer in recent years (the god of storms is in decline). Or maybe there was a monster infested region between two of the nations that has only recently been ‘tamed’, or mountains so imposing that passage was too difficult for any ‘regular’ interaction. Of course deities are also a good way of establishing the differences between cultures.
