In the AI art thread I actually posted several pictures I generated using descriptive keywords from the
Illidan novel, if you want to get a visual for how it would look (Twisting Nether; basalt spires lit with green light; small moons in orbit; small green sun, cratered surface, chromatic stone, etc.).
I also created images of what the cities might have looked like based on those descriptions (black stone streets, obsidian towers lit with green light), also assuming that they would likely incorporate architecture similar to Revendreth (gothic architecture and demonic gargoyles):
I know that "Revendreth but black and green" is basically the way it looks, causing a lot of visual similarities with
Legion, but that is how it's described in the books. I think the Twisting Nether sky and the additions of purples, as well as using greens that straddle the fel and necromantic magic, helps make it look more unique.
It's also a planetoid, meaning it's a small world. Of course, if they were to expand it into an actual playable setting they would no doubt add a greater variety of biomes.
Here are a few other scenery renders inspired by your haunted forest idea:
And some fel-corrupted feral worgen to prowl it: