There are plenty of classes that aren't really the class they are as far as lore is concerned. See Sunwalkers. They're not paladins....they're sunwalkers.
In that same vein a void monster could easily be a "druid" without being a druid, even if we just decided that life energy is anathema to them just for giggles. It still wouldn't prevent a void elf druid existing as like, a nightmare-warden or <pick a name here>.
I mean shit, Undead priests exist and the light physically harms them. So the excuses are there already, and if not, they can be created.
What I would want most though is for those class fantasies to be expanded on. Change Sunwalker spells to use fire and the sun instead of the same light Silverhand paladins use. Change the void elf druid or gnome druid, or whatever druid abilities to reflect the lore for that race/class combination. NOT JUST in the unique forms druids always get, but in every facet of the class. (though class color like orange and icon and stuff would have to be the same.)
But Syn wtf about pvp?!?! you ask? Easy. add a toggle to options so that you can choose "homogenized spell effects". With this enabled, every druid has the same basic nature colors on your client. When disabled, you see the class favor stuff instead.
A void elf that just does the same thing visually as every other druid but with voidy forms is boring to me. A void elf "druid" that shoot nightmare bolts and draws their power from the old gods or just the void directly instead of life? That sounds a lot more interesting.