I guess Blizz is looking rather exactly which race to turn into an AR this time and maybe even if. Frankly, with the new leveling system having more races to play with is really an even bigger bonus, though the lack of more classes is felt(I leveled all classes so far except druids).
I guess that it is seemingly obvious which covenant is leaning to which faction due to important NPCs Venthyr/Necrolords => Horde, Kyrian/Night Fae => Alliance. Another nice split between the covenants is that the former have lost their ancient leader, while the latter not. If there is a big secret to be revealed, or a new order to be etablished it would be the Kyrians and Night Fae who would thus stand for the old, while Venthyr and Necrolords could be more open for change.
Still my view is that the Necrolords simply aren't likely to become an AR. Not just is the leadership split(Draka/Mograine), they don't have the aesthetic to become an AR. This is in the eye of the beholder, but it goes deeper. There are by my count three possible "races" to come from them Gladiators, Normalos(Ossein) and Lichs(Necromancers), all who lack customization options so far in comparision to other models. Even more, they lack a counterpart.
So far, the ARs have been rather symmetrical in theme or size.
Void Elves/Nightborne(elves)
Lightforged/Highmountain(religious bruisers)
Dark Iron/Mag'har(steamtech and shamanism)
Kul Tiran/Zandalari(interesting culture, ships)
Mechagnomes/Vulpera(short/cute)
The lack of real counterpart might be one reason why the alliance didn't get the Sethrak, despite them being aesthetically fresher than Mechagnomes. This makes sense to prevent one faction feeling left out and can be seen by the way Void and Blood Elves are treated. Neither Kyrian nor Sylvar make a good counterpart to the Necrolord Races, not in the way Venthyr and Sylvar do, as they stand for Death and Life aesthetically.
My suggestion:
Stoneborn(Gargoyles) for the Horde and Winged Kyrians for the Alliance.
The normal Kyrians are a bit boring(for me at last), so getting the winged ones from day one would be the better options. Stoneborn armor would be hard to create, true, and they share their style with the Venthyr, but they look amazing. Getting winged ARs would be a nice change from "just" getting another pair and leveling to 30 until you can use these wings doesn't sound so long ... you could even start being able to use them as a special feature.