So, I've actually started thinking in more detail about races I'd like back, and I realized I actually have a much bigger list in mind than I first thought, so I decided to set to it and give my thoughts a chance to flow properly free...
Blasts from the Past:
D&D has had many different races actually be playable in the past, and as obscure as some of these are, I still want to see them return. You know, I didn't realize until actually making this list, but, most of these are beastfolk races. I guess it's because D&D's humanoids have basically amounted to sometimes interchangeable variants of the halfling, gnome, dwarf and elf, the goblinoids (who're playable now), orcs (again, playable already), or just so niche they don't catch the attention (Aventi, for example, are essentially just Tritons with fluff that makes them less jerkholes).
Aranea: What can I say about these guys that I haven't said already? Shapeshifting spiders with sorcerous talents who aren't evil, they just want to be left alone and practice their magic. These were one of the rare races that defied the general D&D tendency to slap on hamfisted stereotype info and call it a day (seriously, the Wallara make me cringe, but then I'm Australian), and I would love to give them a chance to shine.
Lupin: Now, I don't know if we can legitimately have Rakasta return, what with Tabaxi already filling the "catfolk" role. But Lupins certainly have a free slot. And if they brought them back, I'd rather they went deeper than the wolf-riding Great Plains Amerindian culture of their 3.5 revival - it's interesting, but obvious - and took from their source. Bring back the Kingdom of Renardie, of lupins who are urbane and cultured, intellectual and studied with a mighty and developed civilization that spits in the eye of all those who insist that beastfolk can never be anything more than blood-drenched marauders or Stone Age tribals. Besides, are you seriously going to tell me that a jaunty, wise-cracking, devil-may-care, wine-sipping, swashbuckling wolfman paladin isn't D&D?
Diabolus: I don't remember if these guys were my second or first exposure to the potential of Mystara, but either way, they left a huge impact. Like the Aranea, these guys subvert stereotypes, being a race that looks like a bog-standard devil, but who are actually Chaotic Good in nature. Plus, their fluff is awesome; a brave people who fought against the corrupting influence of the Far Realm until their world was doomed and they had no choice but to seek a new home, alien as it is to them, knowing they have no choice but to make the best of it. Plus, they come with instant plot-hooks for any campaign that wants to make use of fighting against the Far Realm - I'm sure you don't need me to elaborate - and make a nice hook for adding "foreign" elements to your setting.
Gnoll: All the potential of the orc, but never explored, especially given the many interesting real world and mythological nuances of the hyena to exploit. 4th edition gave them wonderful fluff that has never been equaled before or since, as a race torn between the black hungers of the fiend and the simple honesty of the beast, tainted by evil yet capable of embracing good. What more do I have to say?
Phanaton: These guys are just goofy fun. Halfling-sized raccoon-monkey-flying squirrel hybrids; there's just something so charmingly silly about them. Plus, they'd make a wonderful forest-dwelling alternative to gnomes or halflings.
Saurial: If lizardfolk are cool, then what's better? How about dinosaur folk! I would heartily advocate getting rid of that dependence on ultra-high sounds and scents to communicate, though, as that's just going to make things silly. As fun as Groot might be on the silver screen, he'd quickly exhaust his co-players at the table.
Centaur: These are actually one of the more prominent "non-evil monstrous humanoids" in D&D lore, and have been important players on a couple of settings, I think. Playable centaurs would be a huge hit.
Voadkyn: These are mostly out of morbid curiosity, to see if WoTC can pull them off now they've made Firbolgs so much more druidic. Plus, it's a chance to fix that godsawful artwork that they had back in AD&D. Seriously, my eyes started to bleed whenever I looked at them.
Pixie: I really don't think they'd do this is any way beyond boosting them to Small size, but still, it's an iconic "adventurous fantasy race", so I think it deserves a shot.
Thri-Kreen: What is there to say about Dark Sun's most unique and well-known of races? As much as their popularity may have been fueled by munchkins drooling over the idea of quad-wielding, they're something new and original, and I'd love to see them back again.
Xixchil: Thri-kreens may be more well-known, but Xixchil are way more fun, in my opinion. Giant mantis people with a racial hat of mad scientist, excellent surgeons and flesh-crafters who consider body modification their religion and who are happy to warp and twist the bodies of others in exchange for the right funding.
Spellscale: This race would be low down on my list of preferences, but still, I'm curious what WoTC would do if they gave these guys a second shot. The basic concept, that dragon-blooded sorcerers sometimes produce a race in which that draconic nature and sorcerous talent are more strongly aligned, whether a throwback or an evolution, is interesting. The actual manifestation of these guys, though... it was bad. Awful "technicolor elf" body-style melded with clunky rules (you literally got no benefits from Blood Quickening unless you were a spellcaster), and terrible fluff that was all about being arrogant, flippant, fickle jerks.
Children of Darkness:
Here's the funny thing about D&D... it's never really tried to investigate the potential of a Dark Fantasy or Horror Fantasy setting. The closest it's come has been Ravenloft, and even, its Gothic focus has always made it, in many ways, even more grounded than conventional D&D - even when White Wolf took a shot at it under 3rd edition, it was pretty obvious that the setting was supposed to be for human players. So, most of these races I'm making up, as they've never actually appeared in D&D in any form.
Caliban: I don't know if this race can come back, but I'd certainly love to see it. Calibans were the only unique race added in Ravenloft D20, cursed and deformed unfortunates born from Gothic spiritual pollution, which made them as strong and tough as they were hideous. Admittedly, a Ravenloft fan made a better version with their "Brutes and Banshees" netbook article, whereas before Calibans were literally nothing more than reskinned half-orcs, but still, there's a lot of potential here for an awesome race.
Mortif: This race appeared in Dragon #313, and it's a wonderfully little creepy race perfect for any horror/dark fantasy game. Better known as Deathtouched, Mortifs are humans who have been literally stained by the taint of undeath, descended from distant carnal unions between the living and the undead. I admit it can go wrong in the hands of the immature, but then, so can pretty much any race.
Dhampyr: Despite being an almost iconic figure - the not-quite-vampire, torn between the dark pleasures of the night and their own humanity - the dhampyr has never appeared in D&D to my knowledge, outside of 4e's experiment with portraying it as a "feat-based race". And that's weird because dhampyrs are quite popular for horror and dark fantasy settings; even the Worlds of Darkness have given them some grudging nods.
Hagspawn: Although I'm using the name from Unapproachable East's "sons of hags who get nothing but their mother's ugliness" race, I picture this as more akin to Pathfinder's changelings.
Blaspheme: We have a (pseudo) werebeast PC option in the shifters. And dhampyrs/vrylokas fill the vampire role. So, what are we missing? A fan netbook for Ravenloft included AD&D rules for playing Dread Golems. And what's the third member of the iconic Movie Monster Mashup? That's right; Frankenstein's Monster. In a dark fantasy setting, I would love to play a man-thing assembled from corpses and brought back to life through black magic, dark alchemy or unholy rites, struggling to define myself as an individual despite my origins and not because of them.
Vryloka: I'm not exactly sure how you'd properly differentiate this from a dhampyr race, but the vrylokas are perfect fits for any dark/horror fantasy setting, and I really want to see them brought back from 4e.
Yokai:
One of my few complaints about D&D's efforts at Oriental Adventures is simple: they've ever embraced the fantasy aspects of an Oriental campaign, always focusing on the "Medieval Eastern" options. The only traditional "Oriental" races D&D has done are Korobokuru (Oriental Dwarves), Spiritfolk (Oriental Elves), and their rather clunky Hengeyokai (a clumsy collection of shapeshifting beasts stapled into one racial statblock) with the D20 version also incorporating Nezumi from the Rokugan setting. Boring! Japanese and Chinese mythology are filled with all manner of fantastic beings, and some are iconic enough that you'd think D&D could at least try to pull them off.
Kitsune: This is, without a doubt, the most famous and instantly recognizable of all yokai. Paizo leaped at the chance to make a kitsune PC race for their Oriental Adventures region, so why can't WoTC take a note from their book?
Tanuki: This member of the hengeyokai family may be more obscure than the kitsune, but at least it's more recognizable than some of the others. If 5e were to do a kitsune race, I'd love to see a tanuki come alongside of it.
Oni: The Japanese ogre/orc is a complex beast; though often a minion of evil, in many stories, they are actually redeemable and even sometimes heroic. This would fill a niche similar to (half-)orc, drow and tiefling PCs in an Oriental Adventures setting.
Planar Races:
As many issues as I may have with the Planescape setting and the Great Wheel, I won't deny that extraplanar campaigns have always fascinated me. It's a legitimate opportunity to explore the weird fantasy side of things, and it calls for a particularly eclectic array of racial options. The iconic Planetouched trinity are back, so a hypothetical "Sigil Adventurer's Guide" would need to cover that ground with other things...
Bariaur: Honestly, these are more on this list for nostalgia factor than anything. The bariaur of old never really struck any particular chord with me, but, I feel that if you're going to the trouble of doing work to support a 5e Planescape type game, you really can't hurt to try and make something out of these guys.
Bladeling: This race is mostly in this list for their depiction in 4th edition, which changed them from yet another "who cares" race of isolationistic xenophobes from the Lower Planes to rebellious/discarded super soldiers now finding an existence for themselves as something other than living weapons.
Githyanki: The githzerai have long held the attraction as a PC race, but honestly, I think there's a huge amount of untapped potential in the githyanki. The only reason they aren't another "edgy" PC option like tieflings is because... well, D&D has long tied itself into knots with their fluff. Seriously, I get that the fact they've taken their obsession with liberty to hypocrisy is supposed to be their thing, but you would think a race with fluff like theirs would have a lot more defectors, simply because you would expect them to realize Vlaakith is playing them for idiots and eating their souls. I always houserule it in my settings that there are actually more "rogue githyanki" than you would probably find "rogue orcs/drow", but then, I like the idea of giving the race the possibility of redemption and revolution against their tyrannical god-queen.
Githzerai: What needs to be said? These guys are one of the original three Sigilite races, and with the recent release of Planescape Torment EE, I figure they deserve the chance to exploit that spotlight.
Rogue Modron: It's a funny thing, really. I'm not a huge fan of a lot of Planescape lore - you'd have to pay me to use the Great Wheel over the World Axis - and I particularly dislike the trait of creating outsiders just to be alignment exemplars. And yet... modrons work. They're completely Lawful Stupid, and yet somehow they manage to be so in a way that's entertaining, in a way the Chaotic Stupid of Slaad isn't. If nothing else, they're a useful race because they generate Rogue Modrons, who have such interesting racial plothooks - why did they go rogue, how do they adjust, what do they want to do now they can distinguish themselves as the smaller, louder *one*.
Shadar-Kai: 4th edition did a lot of good for many races that were kind of dull before, and shadar-kai are a perfect example of that. From cookie-cutter anti-human arrogant fey with a self-mutilation kink to a race of human-offshoots who sought immortality, got it in a way they didn't expect, shrugged, and decided to make the most of it. Shadar-kai are really just an awesome concept, with a lot of development in Dragon Magazine and elsewhere that really highlights their potential. Plus, they're the one race who can out-dark drow and tieflings, but have fluff specifically preventing them from being wangsty emos, so give them credit for that.
Deva: 4th edition just hit so many nails square on the head for me. Devas, they were one of them. I get Aasimar have their fans, and the two races can actually coexist - heck, devas might actually be the primary progenitors for aasimar - but the changes to cosmological lore means I don't know if devas can still exist in 5e without bringing back the World Axis.
Shardmind: I just loved these guys. An entirely new race upon the planes, struggling for meaning in the face of having no history. What's not to love about them, really?