For instance, Aerith. Flower girl by day, goddess by afterlife. I see no reason to limit the Lifestream to straightforward temporal progression, so she could be anywhere. (In Final Fantasy X, we see that the Farplane and dream-Zanarkand and dreams are all connected via the spiritual current/energies of the pyreflies. I tend think of that plane-beyond-life-and-death as "the Aerith zone," i.e. the Lifestream in FFVII).
That was why I had Aerith bump into Auron in the Farplane in my "Going Somewhere" ficbit, which unfortunately went nowhere. I thought they might've made an odd couple like old Doctor Who, back when the Doctor and his companion were not involved romantically and just mucked around the multiverse getting into trouble and putting the world of paradise towers to rights. (Except that Aerith acts like the Timelord; she baffles Auron rather than the reverse).
Auron, as unsent who seems to linger past his expiration date, could appear in dream or spirit planes.
Another is Ellone. Her odd time projection ability would allow her to bring almost any characters together. There was a lovely ficbit by Arglefraster with Ellone speaking to Auron, but unfortunately it's been taken down.
Another is Minwu. I have no good grounds for Minwu to be jumping universes, except that he's one of those characters who gets his own Aerith Zone adventure after he's dead (Dawn of Souls). Also, Sev's wonderful Sight fanfic about Minwu has a complex web of dream/vision tie-ins with other FFs.
Another is Fran. I was intrigued by her ability to open an aerial pathway through the forest, as if opening a wormhole or portal. I realize that in FFXII, that pathway may have been there all along, just hidden until she invoked the forest-magic to reveal it. But what if she really can open myst-paths connecting places where nature itself is aware and alive? That was my flimsy excuse for my Blue Word Lulu/Fran crossover. (Pr0n warning). Cheesy justifications for lemons aside, Fran lives hundreds of years and ... well, what happens to the Viera? Do they go to the Aerith Zone as well?
Another is Rydia, who spends much of her life in the domain of the Eidolons. FFIV presents the Feymarch as a real place, but since Eidolons/Fayth/Espers/etc are spirits, I can't help thinking once again of the dream-world of FFX and the Lifestream of FFVII, all of them existing in that spirit-world membrane between the living world and death. Also, there's the Feywood in Final Fantasy XII where the barriers between reality and myst are clearly breaking down. I would love to see Fran and Rydia encounter each other in that space.
Thanks to time travel, Noel, Serah and Mog could stray just about anywhere. (I loved XIII-2 for the simple reason that it was the first game I've ever played that really explored the possibilities of time travel and alternate timelines). Lightning, also, has landed in the Aerith Zone.
Reaching back to the early games, Final Fantasy I-III and V all deal with Four Warriors of Light, each game with references to previous generations of Warriors of Light and cyclical conflicts. What if the crystals bear the imprints of the previous Warriors of Light like Fayth statues, and that's why they confer "jobs" to the WoLs in III and V?
What if Ingus in Final Fantasy III is the reincarnation of Garland, who was supposed to be the best of the knights before he succumbed to evil? In FFIII, he atones for his previous life, and Sarah/Sara is with him once more.
Finally, there's all the Cids. I've always joked that Cid really is a Time Lord, with busted reincarnations that make him lose his memory. The airship is just his TARDIS!
What FF crossovers do you imagine? Have you written? (And please, grab any ideabunny here you like...consider these prompts!)
Addendum: I recently read Estirose Final Fantasy II ficbit which includes Arc (why not?), and I just stumbled across a Rydia and Auron ficbit by owlmoose that I need to read when I'm not listening to a baseball game.
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