We've been told that Maggie LeFay was, "Brilliant, erratic, passionate, committed, idealistic, talented, charming, insulting, bold, incautious, arrogant-- and short-sighted, yes. Among a great many other qualities." We've also been told that she knew more of the Ways than any other wizard alive, and knew them well enough that she could predict where they would go and when they would change-- something which apparently even very few fae are capable of doing.
Maggie apparently hated the way that humans could be preyed upon by wizards and other supernatural types without it being considered a breach of the Laws of Magic. Like Harry, she probably had the reasons for this stance explained to her, and like Harry, she probably understood it, even if she hated the results of the argument. But suppose that, unlike Harry, Maggie did not decide to drop the subject, or give up on changing things.
Maggie apparently explored the Nevernever in far greater detail than most wizards, and she quite likely went places that few wizards ever go. Suppose that one day she went exploring in some backwater of the Nevernever, someplace no sane wizard would ever go on their own, and she came across a place where passage to the Outside was possible.
We don't really know what the Outer Gates are, except that they open onto the Outside. And we don't really know what the Outside is, except that it's not part of Earth's normal reality. Outside may be one of the far reaches of the Nevernever-- in which case, blocking off access to it seems like it would be next to impossible-- or maybe it's another plane of existence, sort of like the Earthly plane, but quite separate. The latter seems like it would be easier to block off, since it seems like there would be fewer routes between such a realm and Earth. In either case, we know that access to the Outside is rare enough that it can be blocked off, because that is what has happened.
The Outer Gates seem like they must be points either on Earth or in the Nevernever where someone can open a gate and enter the Outside. Even the name implies this: Outer/Outside + Gate/portal. There must be a finite number of them known to the White Council in order for them to be blocked off. But the Nevernever is a big place; it's the biggest place, according to Harry. If the Outer Gates exist there, then it's almost inevitable that there are Outer Gates which have never been found and documented by wizards of the White Council.
So, suppose Maggie LeFay went exploring and found one. She wouldn't step through, of course, because she'd be instantly annihilated by the substance of that plane. But, being who she was, she wouldn't really need to step through to make an excellent guess about what was on the other side. And now Maggie has access to the Outsiders, something no one ever achieves, because the Council is so strict about killing anyone who even looks like they might be curious in that direction.
Maggie goes back to some of her friends, and they sit around, have some drinks, and start talking about how they want to foment political change for the poor vanilla mortals. Maggie brings up her new discovery, and they start pooling their knowledge of Outsiders and what they're capable of. Maybe they go to the Gate Maggie's discovered, and begin experimenting with summoning Outsiders in an attempt to learn more about them. The group eventually reaches a point where they realize that Outsiders can possess people and grant a dramatic boost to the host's natural power. (I'm extrapolating here, based on what we saw of Vitto's amped-up White Court psychic attack. This may not be something that all Outsiders can do, but it may not be limited to a single Outsider, either. There could be a whole class of Outsiders like that, for all we know.)
I have no idea what the group could have learned from the Outsiders to make them think they would be useful in changing the world for the better, but I suspect that that's what happened. I think maybe Maggie had never given up on the idea of leveling the playing field between vanilla mortals and wizards, which would in turn level the field between mortals and most supernaturals. Only, maybe Maggie had given up on the idea of doing it using White Council politics. Maybe she had decided to start cooking up ways to replicate a wizard's abilities in the wider vanilla mortal population. And maybe, somehow, Outsiders seemed like they would be especially useful in doing this.
Way back in Storm Front, we had the Three-Eye drug, which turned out to be a potion that really did grant the Sight to vanilla mortals. Victor Sells supposedly created it, but he was an untrained hack. What are the odds that he could have developed something that no White Council wizard had ever heard of before? Or at least, something that was not common knowledge among wizards. I think Victor Sells probably got the recipe, or the beginnings of it, from someone else. Probably someone in Maggie's original group.
And then we have Kumori, in Dead Beat, talking about eliminating death forever through necromancy. Harry himself thinks that this would do more to level the playing field between vanilla mortals and wizards than practically anything else, making their lifespans equal to a wizard's for the first time in history. If she and Cowl were successful, then combining that with a safer version of the Three-Eye drug, that would eliminate the Sight and the longevity from the list of disparities between vanillas and wizards.
After that, there is the power of magic itself. I have only the vaguest suspicions about how this one might be solved, mostly involving Outsider possession of the entire race of vanilla mortals-- but, honestly, that idea seems way too crazy for anyone to actually want to implement. I doubt that Maggie would've been that far gone. I mean, there's a reason that Outsiders are a banned subject, and summoning roughly 6 billion Outsiders into the Earthly plane would surely seem like a bad idea to anyone with half a functioning brain cell. But, whatever Maggie planned, my guess is that it did involve Outsiders in some way. Not one-by-one possession, but something else. I just have no clue what that something else might be.
Anyway, Maggie gets her group together, which probably included Justin DuMorne, and maybe later Lord Raith, and they begin this grand plan to change the world for the better. I'd imagine Lord Raith's excuse for being part of things was that anything that helped humans stay alive was beneficial to his kind in the long run. If vanilla mortals were strong enough to survive regular feeding by Whampires, everyone would win, from his perspective. Or, at least, that's probably something like what he told Maggie. After all, Raiths feed through superhumanly awesome sex, so if everyone could do that without any ill effects, why not?
Maggie, being brilliant, probably had a better grasp on Outsider lore than most of the rest of them. When things eventually turned bad, which they obviously did, she knew enough to recognize it and formulate a plan to fix the problem she had created. I don't have much of an idea how they turned bad, except to guess that the Outsiders may not have been quite as easy to use without repercussions as the group had believed they would be. Maybe the Outsiders began to exert an unhealthy influence over the group, bending it toward a new, less pleasant direction.
So Maggie left, hooked up with Malcolm Dresden, and gave birth to an Outsiderbane, who was meant to correct all the problems his mother had caused.
I know this seems like a weird and twisted plan, but think again about how Maggie was described: brilliant, passionate, committed, idealistic, talented, bold, incautious, arrogant-- and short-sighted. Maggie could have come up with a plan like this. She could have found an unknown Outer Gate, she could have figured out how to summon Outsiders through it, and she could have decided they'd make a great tool to change the world for the better. And she could have been arrogant enough to think she could use them that way successfully. And maybe she was short-sighted enough not to realize that it would take a dark turn, at least not until the end.
PS: I wrote this post while listening to Gnarls Barkley's "Crazy" on a continuous loop. So if it seems way more nuts than my usual posts, I blame them.