In most Tolkienesque fantasy settings, each race has its own niche. To the dwarves, the mountains. To the elves, the forests. To the orcs, the wastelands, and so forth. Each race also has its own distinct culture, its own archetype.
Except for humans, who are sort of all over the place. If they have their own Thing, it might be Drive and Ambition, or even Diversity - but mostly they are a multitude of cultures (often pastiches of real world societies) instead of being truly distinct like the other races.
But let's try something different for once.
How about this: The world is divided between the different races, with each race having its own territories and several non-contiguous nations encompassing those territories. All races, that is, except for humans, who don't have any real nations of their own. Instead they are barely tolerated by the other races, and most humans spend their lives as wanderers and nomads, similar to the Sinti and Roma of Europe, modern-day migrant workers and similar groups. In some nations, there are cities with large "human quarters". There might even be a few human-dominant "free cities" that came into being thanks to treaties with their neighbors, and whose continued existence depends on good relations with the surrounding lands.
But while most nonhuman races don't care much for humans, they care for the other races even less. Humans have no lands, no armies, and are thus no threat - but they can go where others can't, and thus are useful as merchants and spies. While many humans are poor and only own what they can carry, there are also large extended families and dynasties which have become rich on trade, information, and diplomacy. They might not own much in the way of land, but their coin buys plenty of influence nonetheless.
The premise of the campaign is that the PCs are all humans, or their half-human relatives (half-elves, half-orcs, etc.). Each adventure they travel to a new region dominated by another race, with its own distinct culture and quirks that they had better not run afoul of. They can attempt to get rich by trade with means fair or foul, stealing whatever they can get hold of (and hope they are in a neighboring land before their crime is discovered), or get involved in the intrigues between humans and nonhumans alike.