LostSoul said:
The far realm is the source of the infection. It's chaotic ways have infected the natural world.
The Feywild is, so far, untouched; the Eladrin want to keep it this way, so they kill anyone "unclean" who tries to enter their world.
The dwarves are fighting off the infection, same as everyone else.
You could pick a theme you want to explore, and make that the central theme of the gameworld. Everything follows out of that.
This is actually pretty close to how I would run it if those were my campaign hooks, with just a few alterations (some of them obviously just plot point issues):
I would have the source of the infection a vile entity from the far realm, which is tearing its way through a rift deep beneath the earth. The chaotic energies unleashed from the rift, and the entity itself have been damaging the very fabric of the world for X years. (Campaign decision here. Maybe the world was peachy keen 5 years ago and the infection came quickly, maybe it was over the course of 100+ years)
The dwarves are getting the worst of it. Their underground cities have been ravaged by the earth's plight, as the planet itself rejects the terrible plague within it. Perhaps even lesser abominations have come forth from the far realm in their deepest strongholds, which would give the dwarves more of a clue as to what is actually going on than most would have.
The hardest part is how to handle the Feywild. While I like the thought that the Feywild is uncorrupted and the Eldarin want to keep it that way, it doesn't sound like from any lore that they take much more advantage of it than the inherent ability to move through it in very short bursts. I think you are giving up a great storytelling tool here. Just a first thought here, and it won't appeal to every GM, but how about this:
There is a secret cult within Eldarin society that worships this entity from the Far Realm, and have actively sought to bring it to the material plane for many thousands of years. Perhaps they have tried or maybe even succeeded before in the far distant past, but their attempt was thwarted by an Artifact that only exists in the Feywild. Somehow this Artifact can be manipulated in the Feywild to heal the earth and begin sealing the rift. The cult has secretly infiltrated every level of the Eldarin government, perhaps even to its most powerful leader, while the vast majority of Eldarin society are completely ignorant of their existence. The Eldarin government guards the Feywild under the pretense that they wish to keep it uncorrupted, but in truth they want to deny all knowledge of the Artifact that defeated their plans once before. Well, just the first thing that popped to mind.
If you want to run a 0-30 campaign in this setting this gives you some nice, easy options for an engaging storyarc.
0-10: Fighting localized corruption, discovering the plight of the dwarves, uncovering the source of the world's corruption, and knowledge of the cult.
11-20: Tracking down the cult, infiltrating a powerful cult stronghold, discovery of the deep government infiltration and knowledge of the feywild artifact, confronting the cult leader
21-30: Into the feywild to recover the artifact, deep into the earth's core to the rift itself, into the Far Realm to confront the entity and put an end to its evil machinations forever.
Good luck with the campaign!