I haven't written something for this website in so long! If you remember me at all, I wrote mainly Turks stories, b/c I love them so :D .
DISCLAIMER – I do not own the rights to Final Fantasy VII, Final Fantasy X, Final Fantasy X-2, or anything else related to them. Square-Enix does.
If you know VII, X, and X-2...
This is my interpretation of how Final Fantasy VII and Final Fantasy X are related. If you have read the FFX-2 Ultimania Omega (you can google it… that's what I did, to get a translation), you know that Tetsuya Nomura said there was a connection, beginning with the little Al Bhed being named Shinra. He said that Shinra finds out how to use pyreflies as energy and begins doing so with funds from Rin the Al Bhed salesman. And basically, pyreflies are the same as the Lifestream.
Remember though that these are different planets. If the continents of Spira did plate tectonics to form Gaia (the Planet), that would take millions of years, and people by then would be so advanced, you wouldn't have that agricultural society of pre-Shinra Company Gaia. Unless Jenova wiped out technology like Sin did… but she didn't. ANYWAY, Nomura himself hints at space travel, and he says maybe 1000 years from the kid Shinra's generation, Shinra's descendents will space travel to Gaia. Why would they want to spread their technology to other planets like this? 1) Gaia wasn't inhabited by people in the first place or 2) they didn't know it was inhabited and started off helping with their technology… somehow becoming monopolistic by the FFVII generation of Shinras.
But WAIT! Let's explain some stuff. Number 1 wouldn't work b/c of the following: why would they forget their highly advanced technology? The people in VII, especially when you go to Kalm for the first time, clearly say stuff about how life-saving having electricity is now, as if they only got it recently. Shinra's descendants wouldn't just forget their technology like that, and you have to give time in the history to allow for Jenova's arrival and the Cetra… that would make the Cetra a branch of the Shinra people and I don't like that idea b/c it conflicts with stuff. Unless they reach Gaia and find it just so peaceful that they give up their technology, become farmers, then one day hundreds of years later, someone finds old documents about the technology and wants to revive it and it carries out from there. But really, that's the only thing supporting Number 1.
Number 2, now, makes a bit more sense, but it's still weird. So there were already people inhabiting, including the Cetra. This way, all history could've happened to them too, I mean like cave men and stuff, and that thing in the game's history where some people decided not to be Cetran anymore (however they did that). So then the Shinra come in and start helping out, bringing technology and stuff. As they get more and more supporters, they get more evil. I guess no one wonders where they would have come from, and perhaps they never release their secrets (thus why Cid Highwind thinks he's going to be the first man in space). Why would the space ship they came on not be used for this trip then? Again, hiding the evidence, and maybe they just don't want these Gaia people to succeed. According to Number 2, this would make it seem like it was Rufus' grandfather who made the space trip, b/c really… introducing new weaponry wouldn't take that long, and finding out how to use this planet's equivalent of pyreflies (which should be so similar) shouldn't be that hard either.
Personally, I just don't like the space travel idea at all. I surprised myself with my reasoning for Number 2, b/c I was so against space travel at all, but that really might work out. However, I'd prefer something like the Subtle Knife from the book of the same name, sequel to The Golden Compass or whatever the title is in British (The Northern Lights, I think). As I write, this idea seems more absurd, but I was thinking Shinra the kid finds such a gap between worlds within his own world, and there is no space travel involved. This would mean that the Shinra family's take-over of Gaia happens even sooner, maybe putting Shinra the Al Bhed as a closer relative to Rufus than before, like great-grandpa. Then Cid really would be the first man in space. However, why would he keep it all to himself? He doesn't seem like that kind of person, and Rin of course would either 1) go blabbing or 2) keep it a secret b/c then he'll get wares from that world no one else in Spira knows how to get. Eventually, the secret would get out, as they bring over huge technology to a certain part of Spira wherever that gap is; people would get curious. This does not work at all, and I must dash this plan. Plus, the geography of the world in The Golden Compass is the same as that of The Subtle Knife, which is supposed to be same as Earth. Spira and Gaia obviously don't match up, so if anyone can expand on the Number 2 above (that the planet was already inhabited), be my guest.