Anyhow. Let's get down to business to explore possible subraces and their feasibility. Let's start with the Horde.
Orcs:
Most Orc options can be reasonably handled with reskins. Small model edits could be worked in if you really wanted to go that far.
Uncorrupted: Mag'har have already joined the Horde on Azeroth. WoD will add even more Brown Orc allies in the Frostwolf and whoever else ends up joining. All that needs to be done is the addition of Orc skins in various shades of brown.
Ash Coloured: This includes Blackrock, Dragonmaw, and WoD Shattered Hand all in one. Orcs of this sort have already joined the Horde, and representing them would be as easy as adding various shades of grey to the player skin choices.
Fel Orcs: A good deal of the Fel Orcs on Outland were kidnapped Mag'har who were forcibly injected with demon blood, and many were certainly in a near constant blood rage. Good news is that we've established that killing the source of the blood frees the Orc from the Bloodrage! However, this doesn't change the Orcs color back to what it was before, as seen with Orcs still being green. We already have examples of Red Orcs serving the Horde by the Dark Portal. In short, the lore could easily support former Fel Orcs joining the Horde after being freed from their own Blood Curse.
As for implementing it, there are two ways it could go and it would be nice if both options were available.
One is the simplest route, and simply adding various shades of red to the skin selection. Works well enough for this purpose, and the lack of other features could be explained in various ways. Perhaps these ones were the less mutated Fel orcs, or some of the mutations faded to leave only the red skin.
However, if you really needed to have more than just the skin, that wouldn't necessarily be too hard. Fel Orcs aren't shaped too differently from regular Orcs. Mostly they just have serious dental conditions and a bunch of spikes sticking out in odd places. That's really nothing too hard to just glue on the same way they do hairstyles and armor. Just put the spikes and weird tusks on the Orc model, and set it only to show when the option is enabled. The Fel Orc shape really doesn't differ all too much from the standard other than that.
(EDIT)Mok'Nathal: Bigger Orcs with Ogre skintones. If you're okay with huge Orcs, then okay. I'm averse to large size differences for some reason. But really, it would be easy enough to do.
(EDIT)Half-Orcs: Might require remodeling to add Draenish features, and reskinning to add mixed skintones.
Anyhow, I think that about covers Orcs. There's no other subrace that I'm aware of, and all of the Orcish ones are relatively simple and can easily make sense within the lore. Let's move onto a much more challenging race...
Trolls:
Let's start by saying that Trolls have too much variety. There. It has been said! Now, let's start from the simplest!
Please note, the common lore concept here for how these subraces could be included would likely be Vol'jin doing some recruiting. He hasn't done this yet, so lore would probably have to be developed for that.
Sand Trolls: Sand Trolls have the same build as the playable Jungle Trolls. Implementing them is as easy as adding a variety of sandy skin tones to the available selection. This isn't hard.
Forest Trolls: Unless you want to be lazy and just add some green mossy skins, this is going to be much harder. Forest Trolls have been shown to have a completely different bodybuild, and would basically require a complete remodel to capture properly. However, Forest Trolls are already on the Horde in the form of the Revantusk Tribe.
Ice Trolls: Colours aren't very different from Jungle Trolls, which means that there's no easy way out. Also, their facial structure is noticeably different from that of Forest Trolls. However, that would just be a small detail in the end. If the work was done on Forest Trolls, then it wouldn't be too hard to adapt that body work for Frost Trolls. Only question here is how the females would differ...
Dire Trolls: No. At that point, not even the basic animations would be able to carry over. Also, they wouldn't fit through doors, or be able to ride mounts. Would be fun, but entirely impractical. Like using Vrykul as a subrace of Humans.
Dark Trolls: We have no idea what these are supposed to look like, except for a piece of concept art. Since they were never actually implemented, any design could be used. However, it's pretty much fruitless to examine this option.
Zandalar Trolls: Final option. Could be cool, but it would need different animations at the very least. Probably a different model too. All designed with player customization in mind. Would be lots of effort.
As you can see, there's just too much physical variation in Trolls and it doesn't leave many easy options to work with. Sand Trolls are the simplest choice. Anyhow, significant amounts of lore would have to be created and set to explain practically every single one of these options. But hey! At least there's options! Way too many fucking options.
Tauren:
Ah. Here we have something with a little less overwhelming options.
Grimtotem: Yeah, these are just blackfurred skins with warpaint. This could easily be covered by adding warpaint options. May as well add that to several different races while you're at it. Should cover everyone who wants to be Wildhammer, Dragonmaw, or whatever the fuck else that distinguishes itself through skin-markings.
Taunka: The Taunka have already joined the Horde,and the Taunka model only differs from Tauren in the head department. However, this difference is significant enough that some effort would have to be put into making that new head. Other than that, Taunka would require skins with heavier looking coats. This isn't as easy as any of the options the Orcs had.
Yaungol: Have not joined Horde. Have much thicker fur, which is part of the model. Face is closer to the Tauren norm than the Taunka are, but it's still sufficiently different to be an issue. Unless blizzard already plans to add different facial shapes... Then this whole subrace thing gets easier. Other than that, their horns have a distinct look. They also have unique hair and beard styles.
Overall, most of these options couldn't be handled with simple reskins, and that's just unfortunate.
Goblin:
Even less options than Tauren, and all the options suck!
Hobgoblin: Yeah. No. Not going to work as a Goblin subrace. They're just completely different in practically every way.
Gilgoblin: Would only take some minor remodeling, but it's just not worth it here. Also, there's some intelligence differences and such at work. I suppose if you really wanted to bother, you could find a way to fit these in. Not going to waste time with these. I suppose it could be fitted with an Azshara expansion.
Blood Elves:
Not too many options here, so let's do this!
Late Joiner: Exactly how it sounds. This would be a "political" Blood Elf who only signed back up with Silvermoon after most of the Fel magic shit happened. As such they would have avoided most of the consequences and retained their blue eyes. Very easy recolour options here.
Rehabilitated Wretch: Funny thing is, most Wretched had blue eyes, which is odd if green eyes really are a sign of feeding off too much Fel...
Anyhow, this idea would be more Wretched who have been rehabilitated after the recreation of the Sunwell. However, they would still retain their sickly pale skin tones, tainted eye colors, and various scars from when they were Wretched and took their Arcane or Fel addictions too far. It isn't too hard to imagine some Blood Elves reaching out to help them.
Repentant Felblood:There are venerated heroes amongst the Horde who willingly partook of Demon Blood and betrayed their own people at some point in time. The Orcs as a whole are a Fel corrupted race who spent quite a bit of time murdering innocents for various reasons, all because the leaders they trusted directed them to do so. If some Felblood sought redemption in some way, it would be almost hypocritical not to let them. Especially with how the Scryers and Sunfury were forgiven after serving Kael'thas. Of course, I would have expected them to have degenerated into Wretched after losing their Fel supply. Like how they all go wretched after being drained of their Fel magic.
However, I digress. This would probably work as a sort of variant of the Rehabilitated Wretch. Just one that retains features from being even farther gone before their recovery. On the easy matters, it would just require some added skin colors like red, blue, and dark greys. Oh, and some hair colours too. Skin markings could be covered by that idea I had that such things could be added to all races.
When it comes to model details, the Felbloods are fortunately not too different from normal Blood Elves. They just have a few details tacked on, and tacked on details are relatively easy. Fangs wouldn't be too hard to add, and neither would a few horn variants. The "wings" might be a bit harder, but not exceptionally so. And of course, any of these traits could be toned down for Lore reasons related to no longer intaking massive amounts Fel energies.
Nothing much to say after that. If it were me, I might just give the "Rehabilitated Wretch" idea options to have a few Felblood traits, since they fit together well enough thematically speaking and are both related to excessive indulgence of their addiction. Also, Felbloods become Wretched when they stop feeding, so any potential surviving Felblood are probably amongst the Wretched by now anyways.
Forsaken: Now, I almost forgot this one! Not really too much to say here. I could say I would like darker, more mummified looking, skintones to be added, but that's not necessarily a subrace. So back to subraces!
Undead Elves: These would be Forsaken with Elven features. Would mostly require minor model editing.
Preserved Undead Elf: This one would probably use the standard Blood Elf models as a base, but have undead looking textures. I guess. Don't know how this would be much different from Death Knights... But yeah, would be your basic Dark Ranger/Darkfallen physical types. It could be easily implemented if you really wanted to.
Not much to add other than other undead races. But Elves are the most common ones other than Humans, so I'll leave it at that. Unless you want to play an Abomination or something. I guess Skeletons could work too. Ah well. To get those done with, I think Abominations would basically be creating a whole new race. Skeletons might work a bit better, but would lack variety in appearance.
Anyhow. That's my list for the Horde. I wasted too much time on this, but I think I covered all of the options and how they could make sense, as well as how hard each one could be to work with. Will get to work on my thoughts upon Alliance races afterward.
I really wish I had a good spoiler box option on these forums. One that would make it easier to partition this post. However, as you can see, Orcs have it easiest. Good for them.