Re: Surtur's Role
The "shadowy enemy that predates the universe itself" is definitely not Surtur. There's really only one or two options from the comic book that would fit that description, and they're pretty left field: The Dweller in Darkness, and The Beyonder. Either is a possibility, due to crossing paths with Thor in the comics, but it seems unnecessary to start adding extraneous characters to the plot.
Instead, I'd be willing to bet that the "shadowy figure" is, of course, Thanos; since the next line reads: "In the aftermath of Marvel’s “Thor” and “Marvel’s The Avengers,” Thor fights to restore order across the cosmos." The seeds to Thanos' involvement with Thor and Asgard were already planted in those two movies. Apparently, they'll retcon Thanos' age to make him more ancient than he is in the books, but that may not be such a bad thing.
As for Surtur: the inclusion of Malekith and Kurse clearly point towards Simonson's Twilight Sword saga, in which the Dark Elves' fate is intimately tied with the Fire Giant. I'm sure Surtur will have at least a cameo at the end as a setup to Thor 3, and possibly even be the Big Boss Battle for the end of Thor 2.
I think TDW will show Thanos as the force behind the Marauders invading Asgard at the beginning of the movie; that his attack is just a ruse to distract the Asgardians from Thanos' true intent of stealing the Infinity Gauntlet (back) from Odin's Treasury; that the movie proceeds from there to deal with Malekith and Kurse as the bulk of the movie; and that the end will feature Surtur rising to become the teaser Big Bad for Thor 3.
THE COTTON AVENGERS
...They move like slick cotton on oil.