Which of the various setting specific races do you think deserve a place in 5e?
Depends on what you mean by place.
As much as I'd like it, we can't include everything in the (first?) PHB. So yeah, dividing out the setting specific races is probably the first step. Not the races that have only existed in one setting, but rather the races that lose something of their nature when divorced from their assumed surroundings.
Mul are a great example. I love Mul, but they're very much linked to Athasian dwarves and the Dark Sun world. Taking them away from those elements and giving them as half-human/half-standard-dwarf seems . . . odd.
Take away a Giff's gun? It loses something.
Some others might work better as sub-races, themes or backgrounds. . . kender, gully dwarves, tieflings and aasimar.
Others should be in pretty quick at least. Elan if psionics are going to be core, Thri-kreen, Shifters are another interesting one that work anywhere that has lycanthropes.
I wish I could have voted for the ThriKreen more than once. There is not other insect based race and they are just so darn cool thematically. But thats just personnel biase, I can imagine saying that for most settings they are inappropriate.
I also believe that they exist in at least two settings already, yes? FR and Dark Sun.