Alternative crackpot theory: Square are pulling a double-bluff, and Lightning is in fact dead.
How would that work when we've seen a battle? Easy. A few years back the GBA combined remake of the first two FF games, Dawn of Souls, had brief extensions for both games - in Soul of Rebirth, you play in the afterlife as four of the characters who died. The other extension was called Soul of Chaos. Now take a look at Lightning's enemy in the trailer.
After watching the 15 minute game play demo and the 2:00 official trailer, I have some educated guesses.
First of all, there is obviously more to Serah's dreams than meets the eye. In fact, I wouldn't be surprised if Serah, and her dreams, are the key to connecting alternate realities.
With that in mind, I'm pretty sure Lighting is in fact "dead" or maybe a better word would be "gone" in ONE world, and very much alive in another.
The game play shows a giant monster that seemingly appears out of nowhere, in fact, it looks like even the small hordes "materialize" from somewhere. I think it is one dimension leaking into the other. Lightning lives in the dimension where these monsters are materializing out of, and can't return home without the help of Serah and her dreams.
.. discuss! lol
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Maybe someone saw "Squall is dead", and then went on "We could use this for a new game, since it's just a gender swap!", that's how FFXIII-2 was born (Just kidding) I'm pretty sure she is doing something to the world, or she needed to do something, and told everyone to say to Serah, that she is dead or something. Waiting is too much...
The biggest saving point for FF13 for me was Lightning being the mainish character. It (so far) seems she has been replaced by stereotypical effeminate JRPG guy #41. Still withholding judgment, but I dont really trust SE to do the right thing.
I don't remember which video I saw this in, but a developer said something to the effect of "There's some kind of paradox anomaly or something that makes everyone believe lightning is dead or missing, when in fact she isn't, and Serah is the only one who isn't affected by this, and therefore she knows lightning is alive. So everyone basically thinks Serah is a nutjob.
What I'm wondering is where everyone else is. Snow especially, I'm not sure he would like Serah running around with this dude lol.