The feywilds a big thing in this setting, and entire world parallel to the physical world, with which the physical world often collides in strange and awesome ways. That is the traditional awesome by the way. To be full of awe. It is the kind of emotion evoked by forest fires and tornado.
Knowing that it is going to be these things however does little to drill down into the subject so lets set out what the role of the feywilds in in the setting.
Traditional DnD has a whole bunch of problematic aspects which both from the stance of verisimilitude and any reasonable critical reading of its ethics.
The first of these is simple. The existence of a several dozen sentient species, all pressed right up against humanity can be a very challenging concept, given our world had so far as we know only ever seen a handful of species which bared a resemblance to to humanity. If goblins have lairs just a few miles from town and are less powerful than the militia, why are the adventurers dealing with them instead of the lord or the militia?
Then their is the whole "committing war crimes against a sentient species because you happen to be in resource competition with them and they are evil because we happen to want the same thing as us and at less good at killing us than we are at killing them" thing is a real turn off.
Having a parellel world with different laws(especially ones which account for the existence of the diverse and odd creatures of DnD ) helps to create space(both figurative and geographical) for a wide range of monsters to exist in the world, while not making it seem like goblins and man live right on top of each other. It also allows a way to have less problematic monstrous races, if goblins are not an evolved sentient race, so much as the result of human fears and worries .
Denizens highlight reel:
The Feywild is in this setting is to be home to a wide variety of creatures, some of which will be taking on a very different role in the setting to that which they normally take. Here are two examples.
Goblins- In this setting goblins are a fae race, which spring into existence from childhood fears. They congregate in warrens. They take the kind of sadistic pleasure in the death of small animals than only the most horrible and disturbed of children can muster and gather to to trade the spoils of their raid in markets during the dark of the moon.
Driders- Drow bore me. There I have said it. Like more homogeneously evil species in roleplaying, I have very little time for them, while their use in basically every DnD setting leaves me cold. But driders....they are kinda cool. So driders in this setting are called Ariadrin, and they are the members of the mystery cult of Aria, the Eladrin goddess of weaving and intrigue. Ettercaps are humans who have been captures and corrupted by Drider poison into a servitor race to the Ariadrin.