TL;DR:
Cool story and lore. Lots of hints and assumptions. Some details are off from FFT but most of it fits pretty well.
@Damascar
I checked again on this. If I talk to Jenomis and ask him about the Golmore-jungle he states that it's AT LEAST over 1000 years since his ancestor wrote the Durai papers. My bad, sorry...
Also Jonnycbad is right. We should not forget that Return to Ivalice is more like a "Reference" to FFT than an actual sequel. However, after rewatching every cutscene of Return to Ivalice, reading everything I can read on the Prima Vista and working myself through the story of FFXII Tactics on Final Fantasy wikia, I tried to do an complete storyline of the lore of this Raid. And, except for a few details, it does really fit well with the FFT story.
Please remember since I'm playing FF14 in german and never touched any FF12 title, I might got things wrong or spell names different. If any of you have a deeper knowledge of the lore, please feel free to correct me =D
Ancient Ivalise mythology; very long time ago.
The seven kingdoms Zeltennia, Fovoham, Lionel, Limberry, Lesalia, Gallionne and Mullonde fought a war over territory over hundreds of years. At one point the king of Mullonde gathered a huge army and tried to win the war with ancient, forbidden magic, summoning a demon (possibly Ultima, the high Seraph) to win. However the king could not control the demon and was killed. The demon then aims to destroy the human world, but twelve knights, each of them carrying a Zodiac Stone (Auracite) defeated the demon and return peace to the land.
The original legend of the Zodiac Braves is born.
1200 years before Final Fantasy Tactics.
Ajora Glabados was born and due to miracles he was able to perform, called a saint. However the dominant church of Pharism didn't like that and executed him. Mullonde, the center of pharist teachings, was hit by a tidal wave and sank into the sea.
Ajora was in fact the vessel for Ultima herself, trying to summon the Zodiac Braves. It is possible that Ajora or Ultima cast the tidal wave that wiped out Mullonde him/herself, but thats not stated.
With Ajoras death, Ultima was trapped inside him. Ajora's disciples (unknowing of this) began to preach of his divinity, eventually forming the Church of Glabados.
The church later incorporated Saint Ajora into the legend of the Zodiac Braves. They claimed that Saint Ajora led the twelve Zodiac Braves against a demon summoned by the King of Limberry.
Final Fantasy tactics; at least 1000 years ago (probably much older).
In the land of Ivalice, the Order of the Northern Sky and the Order of the Southern Sky wage a war over the throne against each other, known as the War of the Lions.
Delita and his sister Tietra are two orphans, who were adopted by the owner of the house Beoulve, after their parents died of a plague.
Ramza and his sister Alma are the two children of the house Beoulve.
Delita and Ramza become good friends and attend the military academy together.
One day, they save the life of a lesser nobleman named Argath Thadalfus, who later causes the death of Delita's sister Tietra. Ramza and Delita defeat Argath together, then separate their ways.
Ramza learns that the Church of Glabados has incited the war. Their leaders are possesed by the Lucavi and Auracites and plan to resurrect their leader Ultima, which requires a huge blood sacrifice.
While Delita fights in the Lion War and rises to a hero, Ramza secretly fights against the church and the Lucavi and keeps his erstwhile friend's back free.
After the War of the Lions ends, Delita marries Princess Ovelia and becomes king of Ivalice.
The second legend of the Zodiac Braves is born, telling of Delita and his eleven comrades bringing peace to Ivalice.
Orran Durai, a friend of Delita and Ramza, writes down the "true" story, creating the Durai Papers. But since the Glabados Church does look very bad in it, they declare Orran as a heretic and burn him alive and destroy all his writings, except one which is passed down his family.
30 years ago
Garlean Empire took over Dalmasca. The Bangaa Ba'Gamnan and his men fail to save Prince Rashler and Princess Ashelia. Both of them died. Prince Rashler hands Ba'Gamnan his necklace. The necklace of his sister, Princess Ashelia, will later be in the possession of Jenomis and Alma.
Within FF14:
Alma asks us to find her father Jenomis (Arazlam Durai), who intends to reveal the truth about the War of the Lions and the Zodiac Braves, but also beliefs to find a way to resurect his deceased wife. (There are hints that Ultima spoke to Jenomis through the Otius Auracite and gave him this idea)
We travel to the royal city of Rabanastre, defeat the three Lucavi Mateus, Hashmal and Rofocale and find the ruins of Lesalia underground.
Argath Thadalfus (or an image of him created by the Duma Auracit) reappeares as the final boss in Rabanastre, using the Duma to transform himself.
We manage to safe Jenomis, but the Duma and his copy of the Durai Papers are taken away by Ba'Gamnan and his men.
Later on, the Bangaa enter the Prima Vista and ask for our help saving ther boss Ba'Gamnan who seems to be possesed by the Duma. Ba'Gamnan intends to use the Auracite to get revenge on the Garlean Empire and free Dalmasca.
We travel to the Ridorana Lighthouse, defeat the Lucavi Famfrit and Belias, Construct 7 and Ba'Gamnan, who is turned into Yiazmat by the Duma. We take back the Duma and also the necklace of Prince Rasler from Ba'Gamnan. Both necklaces seems to be part of the same Auracit.
We also discover that the Ridorana Lighthouse was once the Clockwork city of Goug. After a battle between Ramzas party and Construct 6 (Construct 7 in the original game) the latter wiped out the middle part of the tower like city and possibly caused the Ridorana Cataract to emerge. It's also stated that the people of Goug left their home after this and became the ancestors of the Garleans.
Later on, we meet Fran, who leads a dalmascian resistance group and ask her to give us access to the Orbonne Monastry, where we believe Ultima was sealed away by Ramza. But now Alma gets possesed by the Otius Auracite and is teleported to Ultima.
We manage to get to the Monastry, but Ultima tricks us and swaps Alma for her brother Ramza.
Back on the Prima Vista we have a transcending flashback and it is revealed, that Ramza, despite beeing a Warrior of Light, didn't have the power to defeat Ultima and gave his own life to seal her away, until someone would rise to defeat Ultima once and for all. The two necklaces of Dalmascian Royal family were made from an Auracite to lead the future hero on his path. Ramza also wanted to be forgotten by the world, thereby noone would try to mess around with the sealed Ultima or the Auracites again.
Orran Durai wrote the Durai Papers. It's also hinted that Alma Beoulve was pregnant by Orran Durai.
We enter Orbonne again defeat Mustardio, Agrias and Cidolfus (which were allies of Ramza and seem to stayed their as some sort of guardians), find the way to the lost ruins of Mullonde where Ultima is sealed and defeat her. The spirit of Ramza Beoulve thanks us and the characters of FFT have a cameo.
Like Jonnycbad mentioned, there are differences in both games lores. Like in FFT Ramza survived the battle against Ultima, while in FFXIV he gave his life so seal her. Or in FFT it was Construct 7 who self-destructed in Goug, while in FF14 it's stated that it was Construct 6 who launched his weapon destroying Goug and causing the Ridorana Cataract. (Another ungodly like weapon in the possession of Garleans. Just great! What could possibly go wrong? xD )
There are still a lot questions. What was Ultima and where did she come from? Another planet or dimension like Cid assumes? Does she have anything to do with the Ascians? Was Ajora a fallen Warrior of Light? Is Fran's boss princess Ashelia? What about the Garlean enter Goug in the last cutszene?
So many questions...LOVE IT!