Has possibly already been designed over here: Aeran Gideon. Discussion here as to the background of this character.

I designed that, but it was just sort of thrown together. I think we can come up with something better. The only thing that I feel the need to stick is that it's Gideon.

What if the planeswalker helped set up the Aer civilisation.

And when the Aerans mana started failing and they started being invaded, they prayed to him/her to come back, not sure if they would?

And maybe he/she came back after the war started to see that the civilisation had become more beautiful and more decadent, and had to find a compromise between defending the civilisation from the uncivilised monocoloured invaders, and upholding the rights of the invaders to not be mana-starved, which the Aerans were ignoring?

I like Gideon in this role, but I find it hard to "get" what defines each planeswalker. I can sum up batman or superman in a sentence, but when I read a wiki about planeswalkers, it always seems to be a big list of "they did this, then they went there, then they had an argument with them," without a clear idea of what drives them. Is there a better introduction anywhere?

Does the Wizards website still have little blurbs about each planeswalker?

There's http://community.wizards.com/content/forum-topic/3448396 which is pretty good, but only covers some of the planeswalkers. And http://www.wizards.com/magic/multiverse/planeswalkers.aspx but seems to get bogged down in a lot of detail.

I kind of get the impression some planeswalkers are more defined by their cards and others by their novels, but I don't really know which.

Yeah; planeswalkers are annoyingly opaque. My summary is: There's a golem who was on a flying boat with a cat and a goblin and some others. Then there was a big dragon. And then they printed Jace who was just incredibly broken, so they reprinted him again - even more broken. Oh, and there's an incredibly stereotypical red-witch who likes to burn stuff.

...wow - even the short descriptions link launches right into Cataclysmic Comic-book Crossover territory, doesn't it? I guess planes-walkers kinda invite it.

Random thoughts on planeswalkers that might visit:

• Gideon is a martial combatant. I don't think he's much of a spellcaster, though he is labeled as a practitioner of "hieromancy," which Magic creature labels the magic of order but which an internet search claims is "Divination by observing the objects offered in sacrifice." We know from Zendikar and Ravnica that he likes to protect people.
• Kiora worships giant creatures of the depths. She's currently travelling the planes looking for some to put the beat on the Eldrazi, if I understand correctly. She would probably be attracted to our blue demi-plane.
• Little is known about Ashiok, outside of the fact that its a being that preys upon nightmares. Faced with the nightmarish hordes of the demi-planes, I'm sure the Aerans have plenty of fear to spare.
• Tamiyo would likely be intrigued by the odd setup of this plane.
• There are elves on Aer, and Nissa Revane hasn't been seen for a while. Perhaps she would be intrigued by the plight of the Aeran elves. I don't know.

All that said, we need at least one new planeswalker. I believe we spoke at some point of having a 5-color Planeswalker from Aer.

Interesting.

Are there any planeswalkers who like building new things?

So far, Nicol Bolas is the only three-colour planeswalker. A three-colour planseswalker cheaper than 8 would be already quite splashy, I'd vote for exploring that first. If we can think of a planswalker that really justifies 5-colours it would definitely be a very good splashy headliner for the set, but I'm worried if it can be special enough.

I could see Domri Rade affiliating himself with the monocolour defenders, perhaps with a mono-red card.

Tezzeret builds things, as an artificer.

I don't know much about Domri, but I love him for some reason. He does seem a reasonable choice for "passionate" and "smashy" like the monocoloured aggressors. Maybe he could be a fairly cheap card that supports a mono deck in some way as opposed to a gideon-esque large splashy top-end effect.

Yeah, I jumped to Tezzeret as soon as you said "Building things". I could see this as being "A good idea, in concept, that went terribly wrong. And it's creator, Tezzeret, instead of cleaning up his design, just went on to new things, while the plane was forced to find a way to survive on it's own.

Which is... um... a bit ironic...

Oh, yes, and @Vitenka: That's the closest I've ever seen anyone come to answering "What's a planeswalker." You would think decades of explaining what makes a monster cool in the Monster Manuals (Make the complication obvious, and drop it down to one or two sentences.) would have rubbed off on Planeswalkers... but I guess not.

The unfortunate thing is, some magic cards are absolutely dripping with flavour. I want to like Planeswalkers. I think it's a shame that wizards are trying to graft a linear story onto a card game, when just "thinking of cool stuff about planeswalkers" would probably work a lot better.

I see two main problems with walkers.

First, they're mythic rare. Which means that to all intents and purposes, they don't actually exist for me - just as things my opponents sometimes wave around, and are then an overpowered obstacle. This does not make me make friends with them. (If it's a meant to be a theme, put some at common damn it.)

And secondly, the linear thing. Actual planeswalkers, moving from environment to environment - that's just not represented in the mechanics. They're more like a deck box than a card that is played in one game. But avatar cards got dropped, so presumably didn't sell well. Something like that would represent them mechanically better.

Sounds like Vitenka wants the Planechase format rather than planeswalkers as allies with their own spells, which is what the mythic planeswalker cards are meant to represent...

You know, the Planeswalker's do appear on commons, like on Chandra's Fury. It's a shame that the flavor of those cards are often "let's make the Walker look really cool!" instead of "Let's give some insight into how this Walker is different than Jaya Ballard."

well, she is pretty much the reincarnation of Jaya. even their quotes have similar styles.

as for actually walking the planes. that asks for a separate discussion. but simply, World Enchantments do give you that feel of jumping to different locations. What if we could unofficially make Plane a real card type. Then make some rules about Planes jumping--I mean walking. And maybe additional keywords and mechanics to tie in with Planeswalkers. (e.g. remove N counters from PWs you control or something.)

I actually like the planeswalker-affiliated cards, they seem like one of the things that does build an identity for the planeswalker. Not precise enough to distinguish between two planeswalkers whose primary characteristic is "likes fire" but I think you could tell the difference between a Chandra instant, a Koth instant and a Tibalt instant.

That's because Chandra is a pyromancer, Koth is a geomancer, and Tibalt is a sadist who delights in suffering. Their cards might do the same thing mechanically but they'd have a different flavor while doing it.