The last card added is more of a semantic concept, but is definately worth discussing. Rourke mentioned this twice, but I think we're so focused the mechanics that we walked past this flavor point. Still, this may significantly alter what type of mechanics we use. Also, is this supposed to be 5 aliens? Do they all work together to invade, specializing in various parts? Or is this like the Kree/Skrull war in Marvel Comics, where two (five) aliens are fighting each other, and Earth is caught in the middle?

I like that last idea. The five monocolored groups (invaders) are fighting both each other and the native multicolored group. I think the multicolored should be all together, not divided strictly into guilds or shards.

I definitely agree about not cutting up multicolor. We're already making 6 tribes, we don't need 10 - 15.

I've been thinking about creature types for each of the colors. I'm picturing a fantasy world a bit closer to earth than normal, though still with elves and goblins and such, for the multicolored side. Therefore, I'm thinking about creatures that can seem "otherwordly," which doesn't necessarily mean they aren't found in normal magic sets. I do think pumping up the numbers of less common creatures would be good, though. Here's what I have so far, though it's definitely subject to change.
White: Enchantment creatures, angels, spirits
Blue: Some sort of Cthulhu-esque theme? Perhaps with leviathans and kraken? Elementals, also, though distinguished from those in red.
Black: Demons, horrors... probably undead, though I don't like them here. Not vampires. Thrulls?
Red: Devils, orcs, elementals, dragons? Or do dragons belong with the "normal" creatures?
Green: Since white gets enchantment creatures, maybe green gets land creatures. These will have to be fewer in number, though. I'm suck for other ideas, except perhaps some sort of mono-green version of slivers. I don't know why that springs into my head.

Maybe some saproling-fungus love for green? Or has that been done enough?

Hmm... tricky. I think you're on the right track, but we're probably not alien enough. Truth is, in Magic, Angels and Demons are the norm, so most people wouldn't register them as coming from a different plane, no matter how alien they looked.

Maybe it's a mistake thinking about creature types. If this is really a war between 5 different planes crashing in on the populace of another plane, maybe it would be better to focus on what makes those planes so ,alien from this one. The differences should be stark, and not happen in Dominaria for contrast purposes. For example, an entire underwater plane with a Merfolk kingdom could make quite the alien invasion, but it may not hold water (heh) since normal Magic already has Merfolk. We'd have to stretch a little further and find something Magic hasn't done yet, like Sharkmen. That would take people back a bit. We could get even wierder and put the Sharkmen in a Red plane instead, just to mess with people's heads... but that may be going to far.

Maybe we should do what's happening in the main comment board and talk about one color at a time. Also, I'd like to avoid trampling what's happening in white for mechanics right now, so let's move to the other side. What can we do for really wierd looking alien invaders... oh...

I just realized we have another problem. We forgot to ask "Why?". That's probably really important for Green as well, since Green wouldn't normally be 'invading' so much as 'overgrowing'. That would be fine, but it doesn't solve the problem of what is the deal with these 5 attacking planes, and why are they fighting here of all places?

I know this is going to throw things for a loop BUT

How about making Multicolor the natives (integrated society) and the aliens/Invaders Monocolor (focused society that has eschewed/seeks to destroy the other colors).
Then you could have say the green faction come from a world where all life is plant/fungus based, having Plant Elves, etc. You could also work Infect into green (if it isn't too soon to be playing around with it) as the spores they emit to transform "fleshlings" into compost to feed on. Just a whacko idea

This is all kind of tricky. I could see both "The invaders have the same goal" in a similar fashion to how Phyrexia will probably make their next invasion, to "The invaders have 5 separate goals". But, the more I think of the Phyrexian thingy, the more I think we might want to move away from that. That's bound to happen in about 3 years, and is threatening to look a lot like what we're doing here.

I do kind of like a Borg/Phallanx approach to Green. If green was to get invasive, it probably act like a kudzu, spreading out of control, and assuming that any life that can't deal with it, is too weak to survive, evolutionarily speaking. Humans would be more useful as pots and fertalizers, or something for the vines and moss to grow on. It's kind of odd... my mind runs to old Tales of the Crypt comics where strange substances come down from outer space, turning people into plants.. but I don't know if green would do that. Would Green care if you were now a plant? In theory, you might be less useful to green as a plant, since you'll now be fighting for the same resources. I think I like the idea better of plants that force humans to be domesticated... treating them as processing plants to spread their seeds and tend their vegetation. Hmmm... More thinking required.

Please, please, PLEASE no infect or poison. I hate that mechanic with a passion.

I like some of those very alien ideas. I assumed the invaders would be competing for the same macguffin.

I like plant humans and fungus elves. We could have more lignify-type effects that also make something a plant or fungus. Maybe even a "gain control" effect in green.

And the idea of cthulloid stuff in blue. Probably not merfolk, and probably not cephalids or homarids, but maybe more smaller leviathans and octopuses and so on?

Demons, horrors and undead are too normal-black, but perhaps have small demons, so it feels like a whole race of demons?

One way this could work is if there's an outlandish plane full of weird alien things, that's disintegrating or otherwise in trouble. This plane has the magical technology to travel between planes (like the Weatherlight and Old Phyrexia did). So they come and invade the other plane, home to humans etc, which is all adapted to multicolour. The invaders all have a common plane of origin, but they have different factions with their own ideas for how the invasion should work and who should end up on top.

An alternate idea on the story:
The main plain has always been bordered by five alternate planes, each composed solely of one type of mana. This main plane is influenced by the five different types of mana, but the other five have never touched. In an attempt to gain power, a planeswalker draws on these five demi-planes, but things go wrong and she ends up pulling those five planes closer to the main plane. The five one-color planes begin converging with the one five-color plane, and bad stuff happens.

Heh. There's something funny in Alex's approach, that I kind of like. Like 5 alien races from another planet that are still at war with each other, and get enough technology together to start a space race. The multicolored world we're on, is just the unlucky 'America' that the 5 sparring aliens happened to land upon. Or perhaps even a cold war of spy v. spy, similar to the U.S. space race with Russia. Except this time, the moon was already colonized with a primitive culture.

Not to say that that isn't a good plot arc as well, L2i0n0k7. I'm always a sucker for a good "Science gone wrong" morality story.

We have to be careful not to draw too close to the Scars of Mirrodin/New Phyrexia story, which either of these proposed arcs have the potential to do.

True I do like both of the ideas a lot though.

There is also one added bonus to L2i0n0k7's idea. It seems obvious to me that we would end up with a planeswalker, which seems pretty exciting.

That was part of my intent, with possibly part of the story being that his/her spark ignited during the event that caused the planar convergence.