Actually, I have a world where goblins control every major city.
They happened to lose a war very early on in worldgen, and a small number of breeding pairs got taken into the human city nearby.
There, while their human fellows died of starvation periodically when they weren't dying of old age, the unaging, uneating goblins could multiply until they hit population cap, at which point, they just started another human city.
They're actually quite a perfect fit for human cities, especially as there isn't such a thing as disease yet, since they get all the benefits of humanity with none of the weaknesses.
A goblin city in my world has over 10,000 goblins and only 100 humans living in it, while the human civ only barely manages to have more humans in it than goblins overall simply because hamlets are composed entirely of humans (400 apiece). The goblin-human empire, meanwhile, spread across basically half the (medium size) map, all along the ocean.
Most of their cities have at least 5000 goblins living in them, and they cross-pollenated goblins into most of the other city-building civs. It's created a world with several huge cities.
It's also the only 400-year world I have where the cities aren't covered in abandoned shops, since the goblin shopkeeps never die.
I have a save up on DFFD for a bug report, but if you guys want to take the time to explore a max-sized city full of goblins, knock yourself out:
http://dffd.wimbli.com/file.php?id=5842(Note: I've been using my own custom mod to add nagas, so they'll be there, too, although I don't know if that's a turn-off or what.)