(Warning - Potential SPOILERS for X-Men: Apocalypse may well lie below - or they may well not. That's the beautiful thing about a fan theory, no?)
In retrospect, with her subsequent elevation to Hollywood A-List royalty, the casting of Jennifer Lawrence in X-Men: First Class looks like one of the most prescient pieces of superhero movie casting since Marvel took one look at Robert Downey Jr. and thought "Holy Crap, that's Tony Stark." After all, at the time of her casting, she wasn't yet Katniss Everdeen, but simply an up-and-coming indie actress (with only the critically acclaimed Winter's Bone to her name).
And yet, just five years later, she's now very much one of the three key pillars supporting the X-Men franchise, alongside Michael Fassbender's Magneto and James McAvoy's Professor X. That, however, could soon be about to change - as from the sounds of it, X-Men: Apocalypse will most likely be her final film in the franchise.
As it turns out, though, that doesn't necessarily mean this'll be the last we'll see of Mystique. Y'see:
Jennifer Lawrence's Mystique May Well Have Been a Fake All Along
Specifically, her Jennifer Lawrence form (a.k.a. the one just above) may well have very little actual relationship to how Mystique actually looks.
Now, on the one hand, that's pretty glaringly obvious - since Mystique's actual natural form looks a whole lot more like this...
...but there is, as it turns out, a decent amount of evidence to suggest that even that form has been subtly modified by Mystique herself - which, as it turns out, might mean that:
Jennifer Lawrence Leaving the X-Men Franchise Could Lead to a HUGE Plot Twist
Mystique may have never actually looked anything like Jennifer Lawrence's version of the character - even in her blue form. Instead, she might just have been pulling one of the longest cons in cinematic history.
Mystique May Have Been Playing Everyone from the Beginning
Remember that neat moment in X-Men: First Class (above), where the Jennifer Lawrence Mystique...
...Transformed into the Rebecca Romijn Mystique...
...(a.k.a. the version of the character we all knew and loved from the original trilogy)?
Now, at the time, that played like a cute nod-and-a-wink to the original casting, and not a whole lot more. As it happens, though, there might be a lot more to it than that.
Remember the moment in X-Men: The Last Stand, where Mystique was hit by a dart containing the mutant 'cure'?
Specifically, the moment where she transformed into her 'human' self, which turned out to be Rebecca Romijn, only with black hair.
Which is odd, since X-Men: First Class has already told us that Mystique and Magneto are pretty much the same age (we already know Magneto was a little kid during World War II, meaning he can't be all that much older than Professor X and Mystique).
Except, come the present day timeline of X-Men: The Last Stand, she's somehow much, much younger than him.
Which, in turn, is pretty darned weird, seeing as we know she was a little girl, most likely born in the 1930s, when she broke into Xavier's house back in 1944 - making the present day Mystique, even back in the mid-2000s, somewhere in the region of 80 years old.
And yet, she looks like this, even without her powers:
So, how did an 80-year-old woman, fresh from decades of combat, end up looking so ridiculously young? After all, we know that she is, for all intents and purposes, the same Mystique we've seen in X-Men: First Class and Days of Future Past, since she existed in one of that (Jennifer Lawrence-played) Mystique's possible futures.
Well, that's where the plot-twist-y bit comes in:
Mystique WASN'T born in the 1930s
That little kid version of Mystique? As far as the evidence we have so far suggests, she's not actually a kid at all.
Instead, there's every chance that the movie Mystique, much like her comic-book counterpart, is actually far, far older than anyone around her believes - meaning that the little girl we saw appear to Charles is, in fact, as much of a false appearance as any other we see her adopt.
As, for that matter, is her 'Jennifer Lawrence' guise:
In fact, the only version of Mystique we know to actually be real?
That one, back when she lost her powers.
Which, it seems, suggests that Mystique actually possesses unusual - and seemingly non-mutant power generated - longevity, alongside her shape-shifting abilities.
Indeed, it also explains a few large loop-holes in the existing X-Men movies - not least the question of why Mystique would adopt a look so similar to the one Magneto rejected her in as her primary present day cover identity...
...and, of course, just how the junior Mystique ended up in Charles Xavier's kitchen in the first place.
So, What Does That Mean?
Well, potentially, it means nothing more than 'the creators of the X-Men movies weren't really paying attention to their own canon.'
The thing is, though: this is Bryan Singer we're talking about...
...a.k.a. the creator of one of the greatest twist endings in cinema history in The Usual Suspects.
So, even if - as seems possible - the convoluted history of Mystique's background was originally just an error, there's every chance that Singer's going to make it work for himself, and the upcoming X-Men: Apocalypse.
And that means there's every chance that:
Mystique Is Actually Undercover
After all, from what we can tell from the movies, she's an incredibly long-lived mutant with the ability to shapeshift, who one day happened to turn up at the house of a young boy with the most powerful psychic powers on Earth.
And then played a central role in driving a wedge between him and his new-found friend Magneto, ultimately leading him to dampen his powers.
And, finally, after trying to assassinate a bunch of people, ends up mysteriously fishing Wolverine out from the water, for purposes unknown.
And y'know what that suggests to me? It's possible - just possible - that:
Mystique Is Actually Working for...Apocalypse
And, crucially, has been all along manipulating Magneto and Professor X - presumably the only two mutants powerful enough to stop him - into fighting amongst themselves rather than uniting, and becoming strong enough to stop their imminent attacker.
The only difference in the present day X-Men movies? Apocalypse, for whatever reason, didn't arrive. Which, presumably, is why Mystique ended up hanging out with Magneto - she was killing time, waiting for her real boss to turn up.
Or, y'know, she just aged really well...