So a few things I think:
1) Titans, dwarven deities, and the Forgotten Ones are the same thing. Either that or titans are something created by the dwarven deities but I am leaning towards the former.
2) Titans are the pieces of the earth...basically make up the crust/tectonic plates/etc, and lyrium is their blood. We find this out in Descent.
3) I think Arlathan was built on (or inside) one of these titans, perhaps a dead one - maybe as a trophy or just something that the ancient elves or Creators conquered. It's pretty obvious that the Creators had no love for the Forgotten Ones or their soulless servants (that's said somewhere) so let's assume these servants are just the dwarves before their link to the Fade was severed. Let's assume that back then, all dwarves were a bit like Sandal.
4) Somehow, something went wrong, or maybe it was on purpose, but the lyrium became corrupted, or perhaps the titan became corrupted, but red lyrium was created along with the blight (maybe blight first, then lyrium turns red?)
5) **** goes down, blight is spreading, and Arlathan is sealed away in the Fade behind the veil - yes this is implying that the Veil is an artificial construct. Arlathan = the Black City. With the titan sealed in the Fade now, perhaps this is what severed the link between the Fade and the dwarves.
6) I'm not sure how dragons fit into this, but if the archdemons are dragon embodiments of old gods that became corrupted, this might explain where the "call" comes from if it's a blighted version of the "song" that is associated with the Stone.
7) When the magisters entered the Black City, they were pretty much at ground zero for the blight, and so they were hit with it first. This could be why they became the first darkspawn.
8) Keep in mind that all the elven ruins we've been to are mostly underground....if they're all built inside titans (access to lyrium), this might make sense.
9) We know Mythal is still hanging around and she's very interested in collecting OGB souls if the player chose to do the ritual. Fen Harel is also still hanging around as well and they seem to kind of be working together based on the epilogue but it's tough to tell.
At first, I thought maybe the old gods and the Creators were the same thing and that maybe the big plot was Mythal working with Fen Harel to find and free the remaining gods before the Grey Wardens killed them all, but now I'm not so sure. If my above theory is true, then that would make the Creators the big bad of this, and the elves the ironic bad guys of the setting.