A message from 4A itself:
"Artyom’s story may have been concluded in Metro: Last Light, but there are other characters with stories to tell, some familiar locations that fans of the Metro series wanted to revisit, and some new challenges that 4A wanted to explore," said Andrew Prokhorov, Creative Director at 4A Games. "Since completing the game we have been working on these new stories and hope to release them throughout the summer."
That means that while they had some ideas early this year, this is all content that is being created as we speak, and content that will be created over the next couple of weeks and months.
The Season Pass will be $14.99 / £11.99 / €14.99 or 1200 MS Points.
Included are 4 individual DLCs:
The "Faction Pack" and the "Chronicles Pack", which both focus on new and original single-player gameplay and stories that expand upon the Metro: Last Light universe.
The "Tower Pack" which is something different that focuses on challenge and pure gameplay for Metro veterans.
The "Developer Pack" which is not an SDK, but which will give players a unique environment and tools to play around with.
Full details and pricing for each individual Pack will be revealed closer to their release, but combining the current info with 4A's words above will give you some pretty big hints for the first two Packs I believe :).
The Season Pass gives you the DLC Packs in one package, at a discount compared to buying them individually, and also includes the Absats weapon for use right away. The Season Pass will also be available throughout, so there is no deadline for getting everything at a discount; if you want to wait and see what the individual DLCs are first, you can always get it later.
Here is the Absats in action:
The first Pack to release is the Faction Pack, coming in June! The others will follow throughout the Summer. Meanwhile, 4A Games will of course continue work on more updates for Metro: Last Light.