It has been mentioned before, but... I thought it might be a good idea to have this foe easier reference, so...
Anyways, many people think there is ample room in ESO for more races then the "classic ten" which were not quite as classic as many others assume, actually (TES:Arena and TES-II:Daggerfall only had eight, imperials and orcs became playable only from TES-III:Morrowind onward; TES:Battlespire even had only six, missing also argonians and khajiit... and TES:Redguard obviously had only one through the premade character thing, but that doesn't really count anyhow)
...
So, let's take another look at the usual suspects, shall we?
First the ones we have already seen in the game as NPCs, even tho as antagonists (just like orcs were in the elder scrolls series, once upon a time... just like werewolves or vampires are, supposedly...):
Goblins
Despite what some people might say, they are technically possible. They are around, they are (barely) intelligent enough to talk, cast spells and craft stuff, so... they could theoretically be player characters. Thay are also a bit... unwanted... all across tamriel, due to generally being too primitive to follow the laws civilized people live by, and thus usually end up killed on sight or enslaved for "their own good", so it may be a little iffy... but then, we also can make bretons and wander through stonefalls during the DC invasion attempt without getting murdered by our pact allies, so an exceptional gobbo becoming a hero may perhaps not be too far-fetched. It would however be a reasonable idea to give them some racial drawback to represent this bad reputation... like, penalties for vendor business (get less gold when selling, spend more while buying) and bounty reactions (gain double bounty for crimes?)
Maormer
The sea elves would be entirely possible, they are around in this era, they are in the game, they have their own crafting style... all it needs is cobbling together some passives (maybe altmer/dunmer-istic but with storm damage bonus instead of fire?), and there we go. Yes, the race in general is isolationistic, and an antagonist race - and playing as maormer through the AD storyline or the Summerset expansion would be a little iffy. But no more so then playing as imperial in the bangkorai or reapers march questlines, or as DC race in bleakrock to stonefalls, or as AD race through shadowfen... (it might however be plausible to make maormer only playable for DC and EP, unless someone has an certain unlock...
Though perhaps not as proifitable, so I would not count on it...)
Reachmen
They can or can not be their own race, that is up to the developers, the lore would support both them being just primitive bretons, or them being a slightly different race thanks to lots of interbreeding with other people in the distant past... but they are in the game, have their own crafting style and so... easy to do if the powers that be decide they want to. Of course, again there are many questlines in Glenumbra, Bangkorai, The Rift or Orsinium where reachmen are the enemies, but... see above, no more iffy then having an imperial character fighting imperials in other quests or such.
...
...and then some we have heard of, or read about, but not actually really seen them...
Imga
...would be somewhat possible; they are technically around in this era, though they seem for some reason absent from valenwood in ESO, likely hiding somewhere in a pocket dimension like that vanishing village in greenshade (possibly in Falinesti) from Molag Bal and the soulburst troubles no doubt. But that would not preclude some being caught up in the issues and sacrificed by Mannimarco for vestige status before the rest of them got away... and thus becoming player characters (though there might be an inflated cost considering they seem to be a rare sight...)
...
Akaviri
...are not one race but four. There is little lore if all of them were part of the akaviri invasion that took place a mere ten years before ESO - but
lore does say there were survivors from that one, which eventually would make their way to Rimmen in northern elsweyr (DLC possibility?) where they join up with some human akaviri descendants from earlier visits who had once been a strong power in the empire... and make a push into cyrodil somewhen between this time and the rise of Talos in three of so centuries, though they fail to grab the ruby throne for good like everyone else in this time period... so all of them may be technically viable lore-wise... (since even iof the invasion was mostly, say, Kamal like the lore might indicate, they could plausibly have brought allies or slaves of the other races with them) though since they ought to be somewhat rare, it is a little questionable if they would be done (or at what price - adding a surcharge for extra-rarity would seem likely)
Tsaesci
These golden-skinned snakepeople may be possible, depending on how the powers that be choose to depict them in ESO... they -are- supposedly somewhere between argonians and lamias in body structure, so they -could- be done in a humanoid enough way to be viable for all equipment... (and thus as viable player race - the
question if they have legs or not has yet to be entirely cleared by the lore) or there could be two kinds of them, like with the
D&D Yuan-ti where one breed/caste might have a more snake-body lamia-ish body structure, the other a more humanoid one, thus keeping both options open, with the humanoid caste available as player character race (since I doubt anyone would play a race that could not use leg and foot armor slots, yes?)
Tang Mo
The monkey people of the thousand islands would seem entirely possible as palyer race. But they better also come with
polearms to use staves in battle, hear!
Ka Po'Tun
Not much is known about the Ka Po'tun except they are somehow "tiger-like", which could mean anything... a feline race like the khajiit, or perhaps a rakshasa-equivalent with fur-less human bodies and tiger head? Or something half-furred in between? Or perhaps even tiger-centaurs (which would make them unsuitable as player race of course due to non-humanoid body)?
Or maybe they come in various forms like the khajiit depending on moon phases? Its really up to the developers, even though most fans envision them like the tiger-person picture I found... but there is little enough intormation that the poeple at ZOS could do as they wish with them...
Kamal
Little is known about the "Snow Demons", except that they may have been the bulk of the recent invasion that spawned the ebonheart pact, and that they are rumored to freeze solid during winters - something that would make them a bit iffy when it comes to player characters and questing in eastmarch... however, noone can really say if that is not a mere rumor anyhow, it -might- just be indicating a cold-blooded reptilian race that are sluggish in winter and thus prefer to hide in warm caverns during that time...
...or perhaps more liklely ursine (aka, bear-like) race that hibernates...
...or anything, really, and chalk up the legends to their lands being accessible only through mountaijn passes that are simply snowed shut during winter!
Up to the developers.
...
...then there are some "maaaaaybe" races like:
Kothringi
The silver-skinned tribes of black marsh all died out in the Knarhaten flu. How-ever... that only happened twenty years before ESO, so its not entirely implausible that one or two "lone survivirs" managed to hold on, doomed to extinction, and got caught up in the troubles landing them in a coldharbour prison. They -could- be done as PC race, but if so, I would expect them to be as rare as apex mounts... either by hiding their racial unlock in crown crates at the same rarity, ot by charging an insanely inflated price.
Lilmothiit
Are in the same boat as the Kothringi. (not literally, since they for one were not taken aboard the
crimson ship). Very unlikely as PC race, but not entirely impossible.
...
...and then, firmly on the "
nope" side we have:
Dremora (and other humanoid
daedra)
Playing as daedra is way more iffy, lore-wise, then anything else. I mean, come on... why would any quest giver trust a frikkin -demon- to actually help them??? Because that's what daedra are in TES lore! Thus any daedra who shows their face without some sorceror to hold their leash would be seen as mortal enemy, not potential ally. Remember how your stories all start with some sort of rescue? Well, forget that if you were a daedra! You'd get your throat slit before you even woke up after the coldharbour escape... you'd be a dead-ra at best! ;p
Now, yes, it -would- be cool to play a dremora, dark seducer, skaafin or whatever... but it would not fit into ESO. If they ever make a TES game for it though... starting in oblivion, serving your dark masters, going to mundus for questing to avert some dire threat to oblivion itself, facing the usual mobs plus enraged mortals, with adventurer groups as "bossfights"... it could be fun! But it definitely is -not- ESO!
Dwemer
The deep elves are long gone in this era (with
one exception), and they won't be coming back for as far as anyone knows - even in games set a thousand years later, noone has ever recorded seeing another living dwemer (and they would have, seeing how its one of the great mysteries of tamriel, and anyone who found out could have gotten free drinks from the fame along for life).
Only way we will see them is in some "flashback" quest, like the nedes or chimer - which is entirely possible. But never as player characters without some serious lore breakage.
Sload
...are for one not present on Tamriel at this era (Sea Sload incursions into Sumerset aside), since they generally live on isles to the southwest of the continent; for another may be structurally unsuitable as player characters - they are supposed to be sluggish fat slug people, barely able to support themselves on land, which may not be all that great for player characters (I always kinda think of jabba the hutt when I read about them...)
Snow Elves
...are long gone by the time of ESO. And the only remaining ones are in hiding until met in TESV:Skyrim-Dawnguard. Ther blind mutated descendants the
Falmer may show up in some dwemer ruin beneath some skyrim reguin though...
Wild Elves / Ayleids
...have more or less been wiped out by the alessian rebellions, and the cyrodillic empire that rose in their wake; some possibly seeking sanctuary among the altmer... There are rumors occasionally, but those "hidden Ayleids" are kinda like the tamriel-tastic equivalent of elvis sightings... someone's father's brother's nephew's cousin's former roommate has seen one somewhere near someplace, they swear it was true... but whenever scholars go there to check, the reported Ayleids are nowhere to be found (or turn out to be a drunk altmer in torchlight dressing up in a costume on a dare).
And thus... ayleids not really viable as player-race, long gone and only their ruins remain... though perhaps possible as NPC... and rejoice! We meet just one such NPC in the course of the coldharbour assault questline..
Atmorans have become Nords... Youkudans have become Redguards... Nedes have become Imperials... Chimer have become Dunmer... Aldmer have become Altmer...
...making all those races part of history, and no longer around in their ancient incarnation.
As for all giants, lamia, ogres, minotaurs, or other races that have non-humanoid or oversized bodies - obviously those are all unsuitable for PC races (Can't have a player race that would clip through half the scenery after all! Or a player that needs a completely new set of animations!)
(So, I think this time I gathered all the thoughts in one place ;-) )