Ever notice how you see the same shot of Aerith's face in green light at the beginning and end of the game? Obviously, the story of the game takes place within her mind. The segment after the credits featuring Red XIII seems to have "actually" taken place though. However, let's see what we can find out about Aerith from this new perspective:
Aerith is an obvious Purity Sue. She can do little wrong, she has the only Holy materia, her final limit break is Great Gospel, she's Too Good for this Sinful Earth, and her death saves the world. So yeah...
Actually, Aerith is a Purity Sue in Advent Children only, (and Kingdom Hearts series, but then again all Final Fantasy charaters are toned down by this series, to the fans annoyance), and that's Clouds imagination, how Cloud sees her, possibly. She has this Fatal Flaw, which I consider it enough for her not to be in the Mary Sue territory, overconfidence ("I can save Tifa on my own"(from Don Corneo), "I was raised in the slums, I'm used to danger"... "I can save the world on my own, what can possibly happen"). But according to your theory she's a pretty decent fanfic writer(the plot is amazing), despite some inaccuracies.
Sephiroth is a Villain Stu, which is why he is the one to slay Aerith. While Aerith does not quite strike up a romance with our favorite white-haired pretty boy, one can now see why so many fangirls are in lust with Sephiroth. However, an Aerith/Sephiroth shipper would notice that they are Defecting for Love. Zack, and by extension Cloud, are on the other hand merely Aerith's toyboys.
Zack and Cloud, her boytoys? Sure, why not. She always seem to find herself in a Love Triangle, whatever the adaption. But Sephiroth a Villain Stu... he has Character Development (doesn't have to be always positive) through the Compilation, to separate him from a Marty Stu.
Quite a few inadequacies with the world of FFVII and its cultures can be directly attributed to Aerith's laziness in designing the world as well as her fascination with Japan (Wutai), native American cultures (Cosmo Canyon), and large cities (Midgard).
She's creating her own world map, (Midgar is spelled without the "d").
Related to the large cities item, Aerith the Writer clearly knows nothing of farms or animal breeding, as shown by the nightmarish chocobo breeding segment.
More about Aerith personally, she seems to have grown up in a large city, but enjoys natural settings. She seems also to be Christian, or to have been at least raised Christian, since Aerith the Sue hangs out in churches a lot and uses the Great Gospel (see above). If Aerith Gainsborough is her real name, then one can also assume English ancestry (with possible American citizenship).
Or she's Japanese, or simply not Christian at all, when mix-and-matched Catholic and Jewish (The Promised Land) traditions. Oh, and "Aerith The Sue"? You hate her to death, don't you?
Could also explain why, despite the world being completely different than Earth, during Sephiroth's Supernova attack it is revealed to orbit the same sun as Mercury, Pluto, Mars, etc. She's simply imagining the Earth in a different future than hers.
Related to some theories that Cloud is the nexus of Square games (among other things), Aerith, since she presumably created Cloud, is the creator of all of these fictional works. Naturally, they feature the same characteristics of an amateur writer.
The early Final Fantasy games were probably written by Aerith as a child, since they have relatively simple settings and plots.
This must mean that the remakes were really Aerith's rewrites of her old stories.
Kingdom Hearts was obviously a Disney fanfic in which Aerith yielded to the temptation of including her own original works and characters (or Sues). From this, one can glean the enthusiasm that Aerith harbors for Disney properties.
Xenogears was clearly the result of discovering coffee and/or "certain substances" while attending one's first philosophy/comparative religion classes at one's liberal arts school. By extension, Final Fantasy Tactics was the result of a government/politics or history class. Aerith is/was an Arts & Humanities major. She should take more economics classes.
While at her college, she persuaded a few of her friends to create a student film. Though the film was unfinished, Aerith later wrote a characteristic work based on the short film. From the differences in the student film and the analogous scene in the work, one can guess that the other students had some input on the film. One can only guess how good the film would have been had work on it not ceased due to finals. (See the ending of the FFVIII demo disc.)
Final Fantasy IX was an homage to her earlier works and a means of assuaging her nostalgia during a dull summer vacation.
Final Fantasy X was an assignment for her first writing class, assisted by more "certain substances." Naturally, she received a failing grade.
Final Fantasy X-2, on the other hand, was not written by Aerith at all, but by her Brother (capital intended).
