I'm a DA nerd, and now that we have a release date and more info about the game, I figure now would be a fun time to start our relatively baseless speculation.
Quick and dirty background for people who haven't been following the series (meaning, DA2 and the novel Dragon Age: Asunder):
Chantry forms, but has no military arm, way, or will to deal with Mages. The Inquisition develops independently of them, and begins hunting to root out any and all Mages, executing them wherever they are found.
The Chantry eventually initiates the Nevarran Accord, which brings the Inquisiton and all Mages under its control. The Accord splits the Inquisition into two groups - the Templar Order and the Seekers of Truth - and sets up system and network of Magi that operates through the Chantry and is known as the Circle.
The Templar Order is primarily tasked with monitoring and managing the Mage situation, from Circle to Malificarum, and the Seekers of Truth act as both an intelligence organization and as a check on Templar power, as a sort of Internal Affairs unit. The Seekers are unsurprisingly rather hated by Templars.
Fast forward to the end of DA2. Conflict between Templars/Mages has been building for years, with a string of serious conflicts, initiated by both sides. The events at the end of DA2 spark are a huge catalyst for strife, and the Templar Order universally begins tightening its hold on Mages (who are almost universally pissed off about it).
A possible cure for tranquility is discovered. In the same year, Divine Justinia (in reaction to the unacceptable level of Templar aggression against Mages), supports the Orlesian Circle in an uprising against the Templars. When her involvement is discovered, the head of the Seeker order cancels the Nevarran Accord, effectively severing the authority of the Chantry over the Seekers and claiming sole responsibility for the management of Mages in Thedas.
Following this split, the Templar Order begins a march on the Circle at Andoral's Reach, igniting the Mage-Templar War.
Important to note: The Templars have heavily split allegiances between the Circle and Seekers as, unlike the Seekers, they have no central leader. That said, there is a Seeker sect that remains loyal to the Chantry (including Cassandra and Leliana).
As of right now, all we know is that there will be multiple origin options, and they will be human.
You won't play through the Origin, but your background will affect the story. I'm inclined to think this is about establishing your pre-service history, a la Mass Effect, with deeper ramifications. Gaider said it would have a noticeable impact on the plot.
I'm inclined to think you'll be choosing whether or not you come from the Circle, from the Seekers, or from the Templars. I'd also like to see other options, so it's not so obviously class-based. Like Circle/Apostate, Seeker/Bard, Templar/Knight.
What else we know
-It's open world. This isn't open-world like Skyrim, they're just introducing really big zones with a lot of exploration potential. They have NOT called it a sandbox.
-It's releasing on the PS3, PS4, 360, Xbone, and PC.
-New graphic engine (everything in the trailer is rendered in real-time, not cg).
-Besides Morrigan, Cassandra, and Varric, little is known about what characters will be returning.
-Your Warden's story is done, according to BioWare. But whether or not they'll be making some kind of appearance remains open.
-BioWare says they're taking all the features from O/2 that people liked, and building something new. So don't expect either game.
-It's apparently a HUGE world, with one area being as large as all of DA2 put together.
-There WILL be a multiplayer aspect. No word on what that will be.
-Save file transfer will be in place to provide consistency.
-While the game is primarily set in Orlais, you'll be able to travel to other nations (Tevinter cited) to encourage/quell fighting.
Release date, 2014.
My thoughts so far:
-I'm bummed you can only be a human. Andoral's Reach is so close to Kal'Shirok, who definitely has a trading relationship with the Surface. I would have loved to be one of their Dwarves.
-I also wish you could play through the Origin, but I get why you can't. That said, I'm hoping for political origin options. Noble vs. Commoner, the ability to explore the ramifications of being noble and an apostate, etc.
-Multiplayer right now is meh. Though if they did something like Dark Souls, where you could influence other games, it could be cool. I don't mean invading or chatting, but if you could influence the levels of conflict in a region, that could be cool. Like if I ignited all out-warfare in Tevinter, it could have an effect on global levels of conflict (obviously my game, alone, not posing much significance). I like the idea of the extent of warfare having an uncontrollable aspect of sentiment to it.
I fully expect to see Leliana and Alistair in this game. Almost certainly Fenris, if he survives DA2. Isabela is such a fan favorite it would be shocking if she wasn't.