While normal Night elves would never touch the stuff, Illidari night elves make ruthless use of it.
I wonder if playable Warlocks on the Night elf race is actually quite possible if they use the Illidari models that Demon Hunters use. In an ironic sense, his could be the caster version people wanted for the DH, and probably why it never made sense to give it - because an Illidari using fel from range is essentially a Warlock, even though he has a different name.
Lore
Lore wise it would be like how Tidesages are not quite shaman, but on Kul’tirans use the shaman class, or Zandalari druids or Blood Knights are not the regular type of druid/paladin - but it is essentiall the class.
Illidari hate demons, and consider them filth of the universe, but ruthlessly use them. They have scores of demons they employ just like Warlocks, and use them. their order hall shows this quite well and ofc any one who levelled through TBC, played DH or read WotA/Illidan novel and played the Illidan questlines should really know this.
The difference between an Illidari warlock (who should have a different name - like Fel Lord) and most warlocks (most not all), is that they absolutely hate demons though they use their power completely.
When you hear Illidan, you have no doubt he holds demons in utter contempt, and his corruption is using them and their power to destroy their organisations.
Gameplay
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Night Elf warlocks - can only use the Illidari (DH) customisation, i.e. the same model the demon hunter class uses
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Blood elf Warlocks - can have a choice to use either normal blood elf eyes and features, or Illidari ones, because normal blood elves can be warlocks, while normal Night elves cannot.
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Illidari Fel Lord Warlock options, will default Green fire, but you can glyph for the normal fire. The reverse is the case for normal warlocks.