There's nothing before that Fiora dies at the start of the game.  Even after you rescue Fiora the story is still on her because she can't survive for long without her mech and she's still i danger.  The story only took center stage for me closer to the end.

True it does happen rather quickly, but their was the whole part with Mumkar, Dunban, and Dickson. Plus a few missions at the beginning of the game (mostly to establish the friendship with Reyn) before that takes place. There had to be some sort of initial drive. 

Not to say that for YOU it wasn't the relationship, you played for whatever reasons you played. Just saying most played due to the interest of the World being set on two giant Gods, then the various arcs of which most if not all were religious or philisophical in theme.

The biggest one being religion vs science.

Those few missions were to set the back story.  There doesn't have to be an inital drive, many RPGs have a slow start and spend time setting up the back story, RPGs are known for this.  The first real drive is Fiora's death.

The interest in the world set on two giants is meant to hook people into the purchase, but it doesn't keep them playing 10 or 20 hours into the game.  When I play RPGs the back story isn't what drives me to keep playing, its the main objective and for the majority of the game the main objective is focused around Fiora.  The main objectives of the game were first get revenge for Fiora and kill metal face, then rescue Fiora, then find a way to save Fiora from dying, then after the betrayal saving the world took center stage.

I would think the first real drive is the enability to fight the mechon army, therefore fighting to keep the homs race from going extinct. 

Now, if only they had a weapon that could stop them?

What you are talking about is not the drive of the game, but the motivation for the first cast of characters to get involved (plus a little motivation for ourselves). 

*Edit*

For reference, let's look at the story of The Last Story. The drive of the story was for Zael to become a knight so you can marry Calista. That wasn't a side story, that was the main one.

I guess that's where were different, looks like you put more focus on the world and I put more focus on the characters.  

Going back to the topic, going by your logic one can argue FF isn't focused on love stories like you said.  Take FFX for example, I could say saving the world from sin is the real drive, not the relationship between Yuna and Tidus.  Just like your saving the main drive in Xenoblade isn't the relationship between Fiora and Shulk.  So there's no problem with Mistwalker making the next Xenoblade.

If Yuna was the main character I would agree. She could banish Sin. Tidus can't, he was involved entirely because he loved Yuna.

Just as if Shulk could only control the Monado due to certain events I would agree, but Shulk could always use the Monado.

Tidus was the main character because his dad was sin, it wasn't because he loved Yuna.  His dad was the one destroying the world.  My point stands.

He was told early on that he couldn't do anything to stop Sin, so there is a hole in the logic.

Are u trying to make the arguement that Yuna could have single handly killed sin without any help?

Without any help? No

Without Tidus? Yes (at least banish him)