Executive Meddling: Why Vaan and Penelo exist—the original protagonist was going to be Basch, but the developers were concerned that an older hero wouldn't go over well with Japanese audiences, since JRPGs usually have younger, Bishōnen heroes. They become proper characters in their own right in the Gaiden Games. This trope, combined with tightening the loose ends that come with six years of Development Hell is why the International version of the game is markedly different from the original game.
Troubled Production: XII has the rockiest development of any FF game to date, thanks to the collapse of Squaresoft (courtesy of Final Fantasy: The Spirits Within) and its merger with Enix, creative differences with series creator Hironobu Sakaguchi (who resigned and was replaced by Akitoshi Kawazu), and the rough equivalent of $35,000,000 US spent to make the game happen. Five years is a preposterously long time considering how Square had been producing main series FF games at a steady clip of every two years. To put it in perspective, Final Fantasy X was released in 2001, the first year of the PS2's life cycle, whilst the much-promised XII was released in the run-up to the PS3's launch and (At least outside of Japan) after the announcements of Final Fantasy XIII & Final Fantasy XV (Then Versus XIII). In fact, Final Fantasy XV is the only game in the series to have been in development for longer than XII, at ten years & counting as of 2016.
Halfway though production, Matsuno resigned from Square Enix, citing health reasons. He had lost much of his leverage after his project, Spirits Within collapsed like a bad soufflé, and Square Enix wanted to go in a more youth-oriented direction with the game's characters. Considering how much bullion they sunk into the game, you can't blame them for being a bit cagey.
Original design documents and concept art showed that the main protagonist and player character was supposed to be Basch... wearing full armor and a thick, bushy beard.
In addition to Revenant Wings, the game was supposed to have another sequel allegedly titled Fortress. The developers making Fortress went bankrupt, but a few remnants of it survived, such as some concept art, combat storyboarding, and one amazing track.
Writer Revolt: The developers were pretty open about how Vaan was forced as the main character by the higher ups. In the final game, while Vaan starts out as the default playable character, he spends most of the time standing in the background in majority of the cut-scenes and is mostly inconsequential to the overall plot.
Somewhat Vindicated by History as Vaan became popular enough in his native Japan to be the game's representative in the Versus series and later, Final Fantasy Explorers.