This is both an artifact I think should be in the set and a good point to talk about flavor.
I think it was mentioned briefly that it's odd that Aeran humans can fly. These are how. Cards that come with flying, and no wings, would depict these glowing golden rings- or halos- around the human's wrists and/or ankles. I like the image aesthetically and the fact that it sort of evokes angelic imagery, which is good with the general trend toward the Aerans being godlike.
Note that I'm not sure about the costing on this, nor the alternate equip ability.

I like that idea a lot. My question would be how the city floats.

It's fantasy, so I'm inclined to say "it just does." It's not just a city, it's an ecosystem of floating rocks shrouded by clouds. The Aerans (in my conception) aren't even aware that life exists below the clouds, because of how thick they are. In the same vein, the World Below isn't aware of the Aerans.
(I'm coming up with this on the spot. It's not really changing the story, just giving more explanation.)
Perhaps the event that starts the war is a clearing of the clouds.

I assumed there would be limited contact between the Aer and the other races, but I'm happy with floating cities. Perhaps originally they just floated, and over time the Aer found they were starting to decay and started to shore them up, only later realised that by doing so they were leaching mana out of the other lands and it wasn't sustainable.

So, Aer is crashing? Maybe it starts crashing in the second set, as the lands below start to really cut of their mana ties.

Perilous Pitch
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Enchantment
Rare
When ~ is put into the graveyard from the battlefield, destroy all creatures.
"Bring it down! Bring it all down!"
- Gorgophone, Senetress of the 11th Ward.