My personal opinion is that Drangleic is indeed the land of Dark Souls 1 thousands of years in the future, several kingdoms later. I do think that parts of it are Lordran specifically (Did we ever get confirmation that the entire continent was called Lordran? Or just the kingdom of Gwyn?), but not necessarily all of it. Dark Souls 2 is much larger in scope and its entirely likely that we venture out of what was once Lordran and into the other Kingdoms mentioned in Dark Souls 1 (Their future versions, anyway)
-Heide's architecture seems very close to Anor Londo (Minus the blue extras, but that could very well have been part of Anor Londo we've not seen, being a capital city it was likely very large)
-The Bastille is close to the Asylum, Brightstone like the Duke's Archives, and so on.
I actually think what happened was that with the death (essentially) of Gwyn, the humans of Lordran and other kingdoms eventually took over and kept rising and falling in cycles until Vendrick and his brother Aldia discovered the Kiln of the First Flame (The Throne of Want now).
Aldia discovered some way (Considering he was science man) to allow Vendrick to kindle the First Flame once again, starting another Age of Fire which is why Drangliec became the dominant kingdom for so long, Vendrick had access to the power of the First Flame which gave him a Great Soul of his own of some kind which is why he mutated into a Giant of sorts (The Old Iron King has Gwyn's Souls as seen in NG+).
However, Nashandra then showed up (reformed from Manus shards) and convinced Vendrick to wage war against the Giants across the sea (The ones who left with Guinevere) to steal an artifact of some kind. I'm willing to bet that it was an artifact that Guinevere's husband had (Who was Flann the God of Flame) that allowed the Giants to make use of the power of the First Flame to make Golems (Like the Royal Sentinels in Anor Londo and the Knights and such of Vendrick).
So he goes across the ocean and jacks the artifact from the Giants who have since devolved into mindlessness, likely due to their distance from the First Flame and the Lord Souls (Because remember, their souls are pure light souls, just like Artorias who had "nary a murmur of dark" and without being close to the flame they'd deteriorate).
She did this on purpose, hoping that utilizing the artifact and messing with the rekindled First Flame would ruin Drangleic as it had Lordran and corrupt Vendrick which would allow her to finally usher in an Age of Dark. He used it to create Golem armies just has she had hoped, and Aldia began to go insane, exposed to the possibility of such power, leading him to study dragons and Vendrick having to confine him to his manor for the safety of the Kingdom.
Slowly but surely using the artifact warped the First Flame and the kingdom itself accelerating its burn out, bringing about more and more dark, but in a subtler more insidious way than in Oolacile and New Londo. I'm betting that the reason Drangleic didn't go the demon direction is because the goal of Nashandra wasn't to recreate the First Flame, but to get rid of it all together.
At this point the Soul of Life (The Witch's Lord Soul) which seems to have a life of its own senses that the First Flame might be lost forever and possesses a human woman and attempts to use her to create a new First Flame once again.
However, the combination of Vendrick's Golem army and Nashandra shut her down before she can try again and thus cause demons, demons everywhere again and she's confined to Sinner's Rise. The Giants from across the sea then invade seeking the return of their artifact and Vendrick likely makes heavy use of the artifact once again, likely burning down the Flame to a few embers and warping himself further.
They win the war but Vendrick realizes what Nashandra was doing, she was hoping to twist and destroy the First Flame forever, and using an extremely powerful soul (Since hers isn't yet formed) ala Vendrick's new-ish 5th pseudo-Lord Soul create a new Dark Lord and envelop the world in the Abyss.
He flees into the crypt with Vestaldt, making use of Giant's Souls from the war to render him nigh impossible to be defeated by Nashandra and thus used for her plan, at the cost of mutating himself beyond recognition and losing what is left of his humanity. X amount of years after this, Dark Souls 2 starts.
Thoughts people?