What we know about Aer:
Aer is composed of a multitude of floating islands connected by marvelous bridges. The bridges are mainly aesthetic, serving more to keep the islands floating in the same configuration than to provide a place for Aerans to walk, since every Aeran has some method of flight. The islands of Aer settle at the very top of the sea of clouds that bars from Aeran view the ground far below.

I'm not finished, but I have to help make dinner.

The following things are up to be changed (what I said above is also, actually), but I'll state them as facts because that's easier.
Aer is governed by a senate, which consists of representatives from each of the seven* districts of Aer. Each of these districts is named for one of Aer's seven* largest islands, but most also include multiple smaller islands.
The senate building is on Aer's largest island, Tarum, which also houses Aer's financial district and several sculpted parks.
*This number is up for review.

I like the idea of smaller floating clouds. It gives us a chance to make one-shot worlds for single uncommon or rare cards down the line. The fact that Magic is always split into fives always bugs me. It's a good idea, but it bugs me anyhow.

I'd suggest stealing from, of all things, Dragon Quest: The celestials, who live in a world like Aer - are basically incredibly snotty about the ground pounders. They feel sorry for them, pity them, but don't consider them really important. (In DQ9 they protect them and look after them because it's their job - they act towards them like tech support infamously acts towards users.)

I propose that Aer has previously been a non-violent nation that has never fought a war. Petty squabbles, sure, but they've never really had anyone else to fight with. They don't have an army, or soldiers, but only law enforcement and a senatorial guard. This means we have to make Aer feel like they're scrabbling to find ways to combat the enemy.

@Link: It would certainly explain why they were being outnumbered. From their perspective, they've been minding their own business, drawing mana from these planes, when suddenly they're under attack from these unknown forces.

It would be cool if we reconciled the "Aerans didn't know about the existence of the other races" issue. I like the idea that they set up the infrastructure before the other planes were inhabited, which puts the Aerans as a much older race.

Ooh! Maybe Gideon populated the 5 monocolour planes with creatures after the Aerans put down their manastones! And, Gideon didn't know about the Aerans.

The current set of critters seems unlikely to have been created by a white planeswalker. I rather like the idea of Gideon being on the Aerans' side.

FWIW, I like pretty much all of links ideas here. Ceremonial guard, law enforcement, Tarum, senate building, finance, gardens, large and small islands, bridges. Any of those would be good inspiration for cards.

It sounds bad to say, but I like the ideas, too.
I think we need to establish a few things about the story in order to make me more comfortable. I'm a bit confused as to what everyone thinks the Aerans role is in the conflict, and I'd like to propose some ideas as to what I'd like to see from them.

• We've talked about the Aerans being oppressors, but I don't like that. I'd rather have them be accidental oppressors. I think they should genuinely not know, beyond some vague suspicion, about the Gloaming (the planes beneath the clouds). To them, the ire of the monocolored races is sudden and unforeseen. They didn't even know they were taking their mana.
• Aer is a previously peaceful society. They have no experience with war and would rather cultivate knowledge and growth. Any defenses and any military forces they have should seem cobbled together or "mundane." Think police officers fighting a war.
• In order for the conflict to make sense, given what I've just said, we have to start the story with Aer already sinking below the cloudcover, which was previously unpenatrable, perhaps because it's highly dangerous. This means the Aerans should be seen, maybe on just a couple of cards, to be struggling with the pass through this cover.
• Unless I've missed it, we haven't really established how exactly the non-flying monocolor creatures are assaulting Aer, a flying city. Are there portals to Aer that the creatures pass through? Has Aer already touched the ground in places? Is there an outside force helping the monocolored races get to Aer (like a planeswalker that enjoys chaos, or something)?

I propose a bit of a shift in the way we've been thinking about the set, which won't really affect the cards already designed, but merely the way we think about them. What if the first set introduces Aer and the Gloaming in their pristine, pre-conflict states? The monoclor races already have ire against each other, so something like Doomseek can help hose multicolor without it being readily apparent. Things that help deal with flying can be pretty subtle, like tapping creatures, ignoring abilities, or reach, rather than specifically calling out flying. We can't pretend that people won't be mixing multicolor and monocolor, either. With every multicolor card having flying, every player will have at least a few flying creatures.
This means that the second set actually begins the conflict, not the first.

Re: pre war set. I wasn't convinced at first. But maybe at the stage when the Aer have rediscovered the monocoloured tribes, and are frantically exploring their territory in an effort to shore up the mana stones? Then we can portray conflict, and show some "brave Aer explorer" cards and "Aer trying to pretend to be gods" cards but also "overconfident Aer torn down by attacking hordes" cards. The monocoloured tribes could stay approximately as-is, as could the Aer, including "sink into the gloaming" cards, with the exception of not showing any "defending Aer itself" cards.

If we did it right, we could make it shocking in set 2 when monocoloured tribes find a way to Aer and lay waste to it. (And we don't have to figure out how yet -- a helpful planeswalker?)

How do you feel? What did you hope to gain by postponing the war to set ii?

I'm reluctant to plan too much about set ii, because I'm not sure we'll finish set i, so I want to make sure as many good ideas as possible are free to use in the first set.

I only mention Set II because it helps us give a bit of context to something which otherwise has none. I'm not saying I'm confident that we'll ever get around to designing it, even if that would be nice.
I would like the see the first set as the very start of the potential conflict, where the people of Aer just begin to realize where their mana is coming from.

As for the "How do the non-flyers attack Aer" problem, I think the easiest explanation is probably "They don't." It makes more sense, in my head, if the five demi-planes, as a group, found out a way to cut the power back. Aer now feels that it must fight to maintain it's way of life.

If, instead, you wish to maintain a real sense of ignorance for how the world works, you could instead say that the residents of each demi-plane have no idea how they're powering Aer, but the power source has diminished. In this scenario, Aer is sending expeditions into each demi-plane to solve the problem, but the planes are, by their very nature, primordial and violent. The interesting thing about this approach is that we can get very personal with the Aerans, and tell the story from their point of view. The plot itself can feel like it's supposed to be a 'There and back again' sort of plot on 5 different worlds. Unfortunately, as the adventurers explore, they're bound to come to the realization that Aer isn't protecting these demi-planes... it's oppressing them.

That's a lot more subtle a plot than what we were originally suggesting. Instead of massive wars and global shifts in Set II, we'd probably see the explorers of each realm return to Aer, unsatisfied with the status quo. They'd get together and start to do something about it. Probably go public. Then get attacked/arrested.

Anybody like the set title, "The Secret of Aer"?

I do like the idea of the Aerans being sympathetic, rather than cruel deliberate oppressors. If they're the group with humans and elves etc, it'd be nice to be able to be put ourselves in their shoes. (It also helps distinguish them from the black faction.)

It just occurs to me, the obvious answer would be "Aer sinks low enough people can reach it". That accentuates the disaster of Aer sinking, and fits with our suggestion of the first set being set with Aer seeking the mana stones, and the second set with Aer being overrun.

Jmgariepy, with a bit of reflection, I actually like that suggestion more than I thought I did upon first reading it. I just have two issues with it, I think:

• "Aer isn't protecting these demi-planes... it's oppressing them." Well, nobody ever thought Aer was protecting anyone. The Aerans are unaware of the Gloaming and that their main energy source comes at the cost of someone else.
• The idea of the Aerans coming back to Aer and, basically, starting a revolution is very intriguing, but I don't know if it would be as easy and dramatic to represent in cards as the idea of Aer crashing It would be a wonderful subplot in which those Aerans get to say "I told you so" as Aer sinks from the sky, though.

I think you could have both plots running simultaneously, now that I think of it. Aerans who are waking up to what they've been doing could start revolting against their government. At the same time, a full scale invasion from the demi-planes could begin. Truth is, most revolutions are messy affairs... it's never one side against another, like we'd like to imagine it. I'm sure a number of Syrians and Egyptians can back that up.