Halflings, in the current edition, have kind of replaced the traditional gnome role, of nimble tricksters with a cunning and moderately serious streak. Maybe if halflings were geared back towards the jolly, gluttonous, and occasionally malicious "hobbits" of D&D yore, both they and the gnome would have amore secure place in the game again.
Half-orc has, mechanically, been geared specifically towards tanking, and away from its more tradional roles as Thief or Cleric/Assassin. Half-orc seemed a bit more craftly, silent, and deadly when it was first introduced, and not quite so, well, "Warhammer" as it is now.
Half-elf, I think, has a lot of untapped roleplaying potential in the current edition of the game simply because they suck so bad mechanically that no one ever plays them. I'm sure if they get "fixed" in the next edition (possibly by being able to pick the special abilities of whichever race they favor instead of just getting the least of both races) we'll see a lot of players jump on this race, eager for a change of pace.
Something I did with my homebrew setting, btw, was shake off the racial stereotypes a bit, to give the players something fresh to play:
Human: 5 subraces with terrain-specific special abilities: Urbanites, Ruralites, Desert Folk, Woodsmen, and barbaric pict-like folk.
Dwarf: Wiry, dextrous, moustachioed, secretive Foresters with magnificent Viking-style wooden mansions and temples. Favored Class: Ranger
Elf: Tall, aloof, merciless, Usually isolated in high mountain citadels and deep, magically formed halls beneath hills and behind waterfalls. Very xenophobic. Favored Class still wizard.
Gnomes: Malicious, conniving "fallen elves", punished by the gods for their avarice and treachery. Living on the fringes of human lands, grifters with an oddly uncahracteristic ability to craft art and artifacts of incredible beauty. Favored Classes: Rogue or Illusionist.
Lizardfolk: Fallen folk of the once dominant civilazation on the continent, fitting in where ever they can. Worship forbidden Elemental Gods. Noble but tragically misguided. Favored Class: Druid.