Some story shit I made up in ten minutes:
Tifa was engineered by Shinra to kill Cloud and the crew. She betrays them, and Barret dies as a result. This happens after the Dyne encounter. He hands off the pendant for Marlene to Aerith. Tifa gets back with Shinra, and she's a little distressed but still under their control. She wants to know what the next step is, but the Shinra people want to dispose of her because she failed in eliminating them. She fights them off, escapes, goes into hiding, reflects, etc. Tifa single player experience potential.
Skipping ahead: Tifa catches up with the crew right before the famous Aerith scene and sacrifices her life to save Aerith. There could be all kinds of dialogue, cutscenes, switching back and forth between Tifa solo fighting enemies and the crew fighting other enemies, etc. A cutscene where Cloud's very sulky, Tifa dies in his arms, forgiveness, etc.
Skipping ahead again: Zack's with them at the end fighting the Kingdom Hearts Harbingers and Sephiroth. Shit gets real bleak real quick in the battle against Sephiroth, and Cloud sacrifices himself to save Zack. Zack kills Sephiroth, and there's an emotional throwback cutscene to Crisis Core, "Did I live up to your legacy?" Red XIII solemnly howls at the end of it. Zack and Aerith get married, adopt Marlene, and they all live happily ever after.
I don't want to put any more time into thinking what could happen with the other characters. The latest trailer leaves a lot to the imagination, except for the amount of padding. Gonna be a lot of running around and collecting stuff in between fights/drawn-out cutscenes.