Seems like the guys at NeoCore have released new information about the upcoming content. This is quoted directly from the official forums:

"Saxon and Welsh expansions, the Sandbox modes for King Arthur are coming

We’re very proud to announce the release of two King Arthur expansion packs, due in March and April 2010.

Both the Saxon and the Welsh expansions will contain brand new units, heroes and hero abilities as well as completely new objectives and lots of never seen quests.
What’s even more important, they’ll include a new economic system redesigned for a totally new campaign structure, where you’ll be able to play in sandbox mode being either the Christian king or the monarch following the ways of the Old Faith. The expansions will have a complex diplomatic system, campaign winning conditions that can be set by the player and an absolutely free, non-story-based gameplay.

Stay tuned for more info during the next few weeks."