How different would things have been if Tess landed on Gaia during the Crisis Core era?
Hmm, I'm not terribly familiar with Crisis Core (never played it), but I think technically Tess does arrive near the end of the game? She lands a few weeks after Zack and Cloud escape Nibelheim.
But let's say Tess arrives before the Nibelheim incident. She would end up landing in the Nibelheim reactor. Hopefully she'd be able to exit Jenova's chamber or land outside the main doors and make it down the mountain alive, or it would be a really short fic. X) The Nibelheim townsfolk would find her strange, but would probably shrug and think she's another one of the weirdos Shinra sends to the Shinra Mansion from time to time. Someone would hopefully be kindhearted enough to take her in, and once she would learn a bit more about the Nibelheim's history, she would probably want to explore the mansion. Would she find the secret basement? Would she find Vincent? Would she succumb to whatever the heck was going on with the mansion in Figments? Who knows!
(Depending on how early Tess would arrive, she might even end up meeting Reno and the others during the events of Figments. Somehow I don’t think that would go well, what with everyone slowly losing their minds. X)
Eventually, of course, there would be The Incident. Depending on how successful her ventures into the mansion would have been at that point, she might try to join the expedition to the reactor, or maybe even try to negotiate passage back to Shinra in Midgar, to try to learn more about why she would have ended up in one of their reactors of all places. Buuut Sephiroth would still do his thing, and without Reeve's help, Tess's suit would probably have run out of juice by then. If she'd manage to survive the incident regardless, she would probably be rounded up with the townsfolk and subjected to Hojo's experiments. Ouch.
There's a version of this storyline that brings Tess to Midgar, maybe with Cloud or Tifa, but I'm getting too sleepy to think it through. X)