Not sure if that will be lore friendly though
Also, what you just gave isn't logical reasoning. A Highborne is still a Night Elf, just one of nobility. Just like a Dwarf from the Dark Iron clan is still a Dwarf, and one from the Wildhammer clan is still a Dwarf. Any Forsaken Paladin would still have to be a Forsaken, and that idea is still contradictory. Unless you're trying to suggest that we play an entirely different race that's misrepresented by another race in game.
So gnome priests were just convenient to add at the time since the players wanted a healing class. Got ya. Yet of all this, you still are avoiding the very elves who cast out the high elves for using arcane magics. Night elves admonished the use of arcane magic, yet seem to wield it.
This entire thing could end if you would just admit is it "possible". It will likely never happen, Blizz would probably never implement it, for any number of reasons, but are you so fixated on the word "no" and so determined to be right, that you cannot expand your mind for 2 brief seconds to just say "It's possible"? I hope you can. Cause anything.... ANYTHING... is possible, even if it is not probable.
Awful suggestion from the type of people who care nothing about the lore or story or have any interest in it. I'm not trying to generalize here, but it's a given. You'd sacrifice it all because you think 'gnome kittens would be cute omg' or 'tauren rogues would be hilarious' or general 'I want to play this ridiculously lorebreaking race class combo that doesn't make any sense whatsoever because it would be cool' or worst of all, 'I want this race because it has a racial that lets me deal .01% more raid damage'.
There are some race combos that shouldn't exist. Period. Here's a few that shouldn't exist (you'll notice some recurring class themes):
Human shaman. Humans have no desire to be shaman, they are disgusted by the practice, far more than draenei ever were and they don't have any connections to 'broken' humans to teach them nor do they really have the open minded culture to listen.
Human Druids. See above. Yes, there are gilnean druids, they were an extreme case that only occurred in Gilneas, they have none of the background and none of the connections, and most importantly none of the desire to become druids nor 'harvest witches' in any form.
Dwarf Druids. I sincerely hope I don't have to explain why this is a terrible idea. This is against everything in a dwarf's nature, no dwarf would ever want to be a druid.
Gnome Shaman. The most technologically advanced race in the world deciding to use the most primitive practices in the world, 'just because'. No. Just, no.
Gnome Druids. Gnomes have no desire to respect and revere nature and live for its protection and nurturing. How many trees can you find in Gnomeregan? This is even worse than gnome shaman.
Night Elf Paladins. Night elves have no desire to learn to use the 'Light' that paladins revere. They have their own warriors, and more importantly their own beliefs. While not as big a stretch as, say, gnome druids in practice, it is still an extreme case that would be tearing apart their lore for the lulz of playing night elf paladins.
Night Elf Shaman. They have never practiced shamanism nor have shown any inkling of a desire to practice it. In reality it would be almost as bad as human shamans, they likely despise the practice.
Night Elf Warlocks. This goes against everything in their culture. We may as well appoint Queen Azshara as leader of the kal'dorei again and have the entire race bringing the Burning Legion back to Azeroth again. Them barely accepting mages is nowhere near the level of accepting night elf warlocks. Even the new Night Elven highborne are not foolish enough to seek out fel magic. The only night elves that use demonic magic are the demon hunters, and they are a completely different group than warlocks, and will likely be next expansion's new hero class.
Draenei warlocks. This goes against everything in their culture, against the very reason draenei exist at all. If you let them play as warlocks, to have draenei seeking out demonic power to strengthen themselves, you may as well just rewrite the draenei race as demon-worshiping Eredar, because that is exactly what they would be.
Worgen Shaman. See human shaman. No, 'omg they use ghost wolf' isn't a valid reason to debate. Gilneans are the most sophisticated, proud, and narrow-minded human race on Azeroth -- they are even less likely to desire to be a shaman than Stormwind humans are, and that's saying a LOT.
Worgen Monk. Worgen are savage fighters, learning to fight through applied pressure and specialized styles would seem redundant to most worgen, in addition to the fact that their long claws and shape of body would make learning kung-fu all but impossible.
Orc Paladins. Just, no.
Orc Priests. See above. I am not going to waste my time explaining either of these. Two NPCs in a single 5 man dungeon that have generic heals does not mean the entire barbaric orc race wants to start swinging the Light.
Orc Druids. Orcs have no desire nor inclination whatsoever to become druids.
Troll Paladins. Just, no. The primitive and shadowy worshipping trolls will never become Light-wielding vindicators. Period.
Tauren Rogues. One of the extremely few cases where physical traits prevent the validity of a racial choice. Tauren can technically theoretically learn to become rogues, but their size and form would make it extremely unideal, they would be making mistakes constantly that they could not prevent. In general, it's just downright ridiculous, which is ironically the only reason people even want this awful class combo.
Tauren warlocks. This goes heavily against their culture. Just because some grimtotem, the scum of the tauren race, choice to learn demonology, does not mean that the rest of the revered tauren race would even consider touching such a foul practice for personal magical strength.
Undead shaman. Not only do they have no desire to practice it whatsoever and are likely disgusted by the practice, but it goes against their very nature.
Undead druids. One of the worst and most ridiculous possible class combinations, second only to draenei warlocks. Druids are about nurturing life and rebirth. Where exactly do you see this in undead nature? In their lands? In their cities? In their culture? Anywhere? This is like the polar opposite to their very being.
Blood elf shaman. Very similar to other races on this class, such as humans, but at a much greater degree than any of them. One of the worst possible class combinations, and absolute worst for shamans.
Goblin paladins. Do I really need to explain this?
Goblin druids. Please see above.
Pandaren death knights. An impossible choice, given that pandaren didn't even really show up on Azeroth until long after the Lich King died.
Pandaren Paladins. There's absolutely nothing about this that would work. Pandaren have extremely different beliefs from the rest of Azeroth. There is no reason why pandaren would ever become paladins.
Pandaren druids. Not quite as big a stretch as the above two, but still far past the point of breaking lore. Druidism is an extremely specialized skill taught to specific races. Pandaren neither have the inclination nor the desire to become druids.
Pandaren warlocks. Against their entire culture, though for a different reason: The sha. Warlocks practically embody negative emotion through their lust for power and general lack of respect for life.
On top of all of these lore breaking options, most of these recurring classes would require new models for each race including: Shaman, paladins, and druids especially.
Now, while I think that many class combos would break the game, I think there are a few that should in fact be added to the game. I'll give a brief list:
Gnome Hunters. Affinity for nature is not a prerequisite for becoming a hunter, or else undead would never be able to become them. Being 'too small', ridiculous as such a debate would be, also wouldn't apply or goblins (and dwarves for that matter) would be out of luck. All that is really needed is the ability to pick up and fire a gun and bow, and every race on Azeroth can do that. More importantly, gnomes are some of the most skilled marksmen on Azeroth. It makes no sense for them to not be able to be hunters.
Draenei druids. A bit of a stretch and would require some lore adjustment. It would bend lore, but not break lore as draenei have an open mind, an affinity to nature, and close ties to their night elven allies. This race is also different enough from the other druid options to be a good alternative choice.
Worgen Paladins. There's no reason worgen paladins shouldn't already exist. There were gilnean paladins already. This change wouldn't even require new mount models -- they use the same mount humans (and dwarves) use.
Undead paladins -- OR -- goblin monks. The former would be a self-loathing individual of constant pain and anguish -- their motives would be a mystery, especially in serving the undead. They can exist, but the agony such an individual would have would be unbearable. This is right on the border of lore breaking, but not quite there. Goblin monks, on the other hand, would be implemented a lot more simply. There's little holding back their existence except for an excuse for a pandaren to show up in Kezan. Their interest in discipline is a bit of a stretch but not a terribly big one. I recommend goblin monks.
Blood elf Druids. At a glance it may seem like a stretch far past the breaking point, but it's actually not. Night elves are druids, and blood elves share their deep respect for nature -- the farstriders do, at least. While this would be a much bigger step for them than night elves becoming mages, it's feasible. Even better is the fact that their shape shifts could potentially be reskinned night elf forms and work perfectly well. Blood elves would make for a very diverse tertiary option for horde druids.
The reason I think Horde should get 2 and Alliance 3, is that they are already behind a class combo. If for some reason you think it 'isn't fair' you could potentially add tauren elementalists (mages) as an option for Horde. Speaking of which, I did not forget any class combos -- those I didn't mention are possibilities, but stretches and (in my biased opinion) uninteresting ones.
My two cents on this topic.
I'm aware some class/race combos sound very, VERY stupid. Like Undead Druids. What I said before literally was off the top of my head, but at least I actually attempted to give reasons. And Undead Druids wouldn't be like the shapeshifting druids of the kal'dorei, they would be more like apothecaries that grow the plagued flora themselves. Even so, the game mechanics requiring undead druids to be able to shapeshift as well would be odd. And oh yeah, Wildhammer Druids? They exist in the lore of the RPG. I'm now aware it isn't classified as canon, and I apologize for not knowing that before, though it's still there. After all this entire thread is speculation, so if the removal of race/class restrictions would happen, why not make RPG lore canon rather than making up something completely new to add to the game?
-From WoWPedia.Wildhammer dwarves have close ties to nature. Many are shamans, and some are druids. A few Wildhammers revere the Holy Light, but the faith demands too much organization and philosophy for the comfort of most. Some Wildhammer dwarves in Kalimdor have also been studying with the night elves to learn more about Elune, the moon goddess.
It's interesting to think about, at least.