A Non-Human World (DMG)

The DM can, if he chooses, make any class available to any race. This will certainly make your players happy. But before throwing the doors open, consider the consequences.
If the only special advantage humans have is given to all the races, who will want to play a human? Humans would be the weakest race in your world. Why play a 20th-level human paladin when you could play a 20th-level elven paladin and have all the abilities of paladins and elves?
If none of the player characters are human, it is probably safe to assume that no non-player characters of any importance are human either. Your world would have no human kingdoms, or human kings, emperors, or powerful wizards. It would be run by dwarves, elves, and gnomes.
This is not necessarily a bad thing, but you must consider what kind of world non-humans would create. Building a believable fantasy world is a daunting task; creating a believable alien fantasy world (which is what a world dominated by non-humans would be) is a huge challenge even for the best writers of fantasy.
What would non-human families be like? What would the popular entertainment be? What would non-humans value? What would they eat? What would their governments be like? A society governed by nature-loving elves would be a very different place than a human-dominated world.
It is possible that certain character classes might not even exist. Paladinhood, for example, could be a uniquely human perspective. Would elves and dwarves hold the same values of law, order, god, and community to which a paladin aspires? If you only change the image (i.e., have elven paladins behave exactly like human paladins), what you've got is the "humans-in-funny-suits" syndrome. Even within the human race there are vast cultural differences. Think how much greater these differences would be if the blood were entirely different.
Also, if humans are weak, will the other races treat them with contempt? With pity? Will humans be enslaved? All things considered, humans could have a very bad time of it. If, after considering all the potential pitfalls, you decide to experiment with non-standard class selections, do so carefully. We offer the following advice:
Allow nonstandard race/class combinations only on a case-by-case basis. If you institute a general rule—"Gnomes can now be paladins"—you will suddenly find yourself with six player character gnome paladins.
If a player desperately wants to play a gnome paladin, ask him to come up with a thoughtful rationale explaining why this gnome is a paladin. It must be plausible and consistent with your campaign setting. If the rationale satisfies you, allow that player, and only that player, to play a gnome paladin. Explain to the other players that this is an experiment.
Don't allow any other gnome paladins in the game until you have seen the first one in action long enough to decide whether the class fits into your game. If it does, congratulations—you've broadened your players' horizons. If it doesn't, don't hesitate to tell the gnome paladin player that he has to retire the character or convert him to a normal fighter. Never allow someone to continue playing a character who is upsetting your game.
By following this simple rule, you can test new race/class combinations without threatening your campaign. Moderation is the key to this type of experimentation.