If I recall from one of the interviews that Yoshi gave. Tanaka had already developed the main story scenario up until when the main story finished. I don't remember if this was around level 30 or so or further. I remember Yoshi saying that there was not a good way that he could progress the story from where the main scenario was leading so he decided to make it a part of the GC storyline instead when he took over.

I know for certain that the final couple of battles in the GC series (United We Stand, and To Kill a Raven, possibly a couple of other battles maybe primals) were battles that Yoshi oversaw.

I know the armory system and job systems have always been there from the start of 1.0. The idea was that if you ran around solo, then you would be running around in your class with full support from other classes. But if you grouped up, there were more benefits to changing to specific job to fit a specific role in the group.

If I recall the original idea of 1.0 was to be FFXI-2. The idea was that you could go into your mog house and select another world and come to FFXIV; hence why the races were similar (This was from an interview with Tanaka I think). They found out that the PS2 could not handle this operation so they dropped it and made it into a separate game. The races being similar were to draw the FFXI crowd into the game to give them something familiar hold onto.

I don't believe the gold saucer was a part of the original idea when 1.0 was conceived. I think only after the failed launch of 1.0 did SE want to start trying to incorporate aspects of other Final Fantasy Series into the game.

I don't know how far Tanaka had planned out, but I know that 1.0's world could have supported a flying version being that it was an open world. The only problem with that was the memory limitations of the PS3. Hence why when Yoshi took over he split up the cities and worlds into segments. But having said that in 1.0 there was not such thing as jump and there were invisible walls and snags that your character constantly got caught on, so take that for what you will.

I remember Yoshi saying that the 1.0 developers really didn't have a direction for which they wanted to go. They just wanted to push the graphical qualities as far as they could, hence the high specifications for the game when it came out.