Okay, so I was just thinking back to when Dungeons & Dragons 4th Edition came out and there was much wailing and gnashing of teeth over the fact that gnomes had been removed from the pantheon of playable races and relegated to the status of "monster". And to add insult to injury, they added tieflings and dragonborn. (There's an amusing video on YouTube about this: http://www.youtube.com/watch?v=4UqFPujRZWo) Oh, the humiliation! All the gnome fanboys and fangirls were devastated. And Outraged. Devastated and outraged, I say. I, on the other hand, was like "Meh, nobody plays gnomes anyway". I was actually quite pleased to see the inclusion of tieflings and dragonborn as player races right up front. I mean, this is a fantasy game, isn't it? Yes, there are elves, dwarves, and halflings, but those are really so humanoid that they really might as well be humans anyway. If it's a fantasy campaign world I expect there to be really cool definitely-not-human races to play.
But back to the stupid gnomes. So as I was saying, nobody plays gnomes. (Okay, I did make one for a Pathfinder Society game, but that was sort of as a joke.) In all the games I've played in, lo these many long years, I have never (that's never, as in, um, never) seen anyone play a gnome. Nobody plays dwarves either--I've seen exactly one dwarf PC in 30+ years of gaming. Halflings have been quite rare and only played by people using them as a min-max rogue choice to get the racial bonuses--that's where the idea of Kender came from.
For the campaign world I'm building I've decided that the core ("Elder") races are human, dwarf, and elf. The other three "core" races from D&D appear only as the result of matings between the Elder races:
Human + Elf = Half-Elf
Human + Dwarf = Halfling
Dwarf +Elf = Gnome
Other core races for the world will include tieflings and aasimar, dragonborn, and the demi-elementals (ifrit, oread, sylph, undine). The demi-elemental types will have to designate one of the three core races as the non-elemental component of their parentage.