@Althenar
High Elf druids/shamans could make sense of Blizz decided to write it like that, there's been countless suggestions for this implementation that would make sense. Its essentially all head canon for now though
I think Alliance High Elf DK's probably wouldn't be playable, as they would likely be slain High Elves who died defending Quel'thalas when Arthas sacked it, and we know for a fact that those DK's joined the Horde. Then again, Wargen are able to be DK's, so anything is possible. They could be drawn from Silver Covenant members who were slain during Wotlk
The only high elf death knight in the horde that we know of is koltira. Like you said, blizzard could write it like that so they are playable.
Lorewise, HE DK's are not an impossibility, but they would be escarce enough that maybe they wouldn't be playable -similar to how paladins and warlocks can be seen as individuals, whether they are part of a larger trend or just outliers just depends of the context of the hypothetically playable HE group.
As for HE DK, they would have to come from the same source this hypothetically playable HE faction, and if we put the SC in that place it is certainly plausible that a considerable number of SC fell during the first incursions into Northrend, or even more "Neutral" Kirin Tor High Elves in the same way that would have gravitated to the alliance having to pick a side.
Like timeline wise they are probable, but again, the issue is if their numbers would be enough to be a playable race. If we look at things from a Silver Covenant perspective, classes linked to both their mage and ranger branches make sense:
Hunter,
Rogue and
Warrior as sections of the more Farstrider inspired military. We see ranger types everywhere, but we also see warriors and rogues as Silver Covenant Tropes on Isle of Thunder and the Purge of Dalaran
https://wow.gamepedia.com/Silver_Covenant_Enforcer
https://wow.gamepedia.com/Silver_Covenant_Vanquisher
https://wow.gamepedia.com/Silver_Covenant_Captain
https://wow.gamepedia.com/Silver_Covenant_Assassin
https://wow.gamepedia.com/Silver_Covenant_Agent
We also have the spellbow, which seems to be the perfect blend of their Farstrider and Arcanist background.
https://wow.gamepedia.com/Silver_Covenant_Spellbow
As for their
Mages, we also have them as very commonplace, as much as their rangers (Warmages, Battle mages, Guardians, Sentinels, Arcanists)
I would add
Priest making this "baseline" cut, since they are one of the iconic War3 units, the need for a healing spec, the fact that those War3 mages has been codified as "Mage-Priests" which makes them part of the mage branch as well, yet I have to say the Silver Covenant itself doesn't seem very light focused at all, so in a way, Priests could be on that same "outlier" field than paladins and warlocks are -based on the SC trainers in Dalaran BTW) the difference is that they have a lot more of history.
Warrior, Rogue, Hunter, Mage and
Priest -with the last edging on that last place- I think it's what the Silver Covenant gives us as a group, so I would call these the
very likely options.
Paladin and Warlock both have members on the Silver Covenant, but merely one of each. Personally I would put Paladin as a bit more likely lorewise since there are more High Elven Paladins out there, just not affiliated with the Silver Covenant, where as Warlocks truly seem to be a one man thing -cause we are not counting warlocks in decidedly evil factions like the Burning Blade or the Scourge-
So
Paladin could be seen as possible given that they exist as at least a handful of NPC's, but not as part of the SC, so it's a matter of expanding of that.
But beyond this, every other class would be an "out there" choice, meaning that it would need a very specific justification as to why they are now part of the SC, let alone as a group by themselves.
Getting them out of the way first, we can give
Monk a possible rank since their education is commonplace, and we have seen HE monk NPC's. Monks are the only class that makes sense everywhere, since all it requires is new people willing learn the discipline.
Back to
Warlock and
Death Knight, I would have to mark them as "unlikely." The former might very well just be an outlier and DK's are plausible, but unseen.
Now,
Druid and
Shaman are the two classes that would be completely out of the left field, with nothing going for them. So that would be a No. Yet we can't dismiss these are the two classes that would give HE their own flavor and unique choice pool, so from a design perspective, they could be the most interesting. Again, there is no lore to speculate about these classes, but that's not the same there are no possibilities. We can't dismiss that High Elves come from Night Elves, who are a very druidic race, and that Ranger's themselves can be so connected with nature as to have some druidic powers and affinities. And as for shaman, we have the Highvale as an enclave of elves that work very closely with shamans, and as the most nature leaning group of High Elves, it is certainly not unconceivable.
Note this breakdown is from a SC perspective, but while they are not the only group of High Elves left even if they are indeed the most notorious. The Highvale themselves are rather unique on their both naturistic and light based faith, and as a basis they have a lot of potential, but they are indeed very un-explored. We also have the Allerian Elves, which while hey would have a very similar make up to the Silver Covenant in militaristic terms, they wouldn't seem to have much in the way of magic branch, since the Alliance Mage HE would have ended on Kirin Var. And while SC elves manage to live within a more combined human/elven context to the point the SC itself is a predominantly elven, the Allerian elves certainly live on a far more human dominated one, so we would have a more soldier like, outdorsman and human inclined group in them. Also probably a bunch of 18+ Half Elves.