According to the announcement, the add-on will contain +10 hours of gameplay (how do they measure that, anyway?), a new dynasty weapon (if you like that sort of thing) and more artefacts. I am also told that the actual writing this time will be done by Julien Pirou (aka. "Marzhin"), which should at least improve the quality of the story compared to that of the campaigns.
Also in the announcement: Patch 1.5 is scheduled for July 12th and will feature the Town Screen. What is particularly funny is the wording of this on the official page:
"Did you know that the very first Town screen appeared in Heroes I and became an important hallmark feature? Town screens are one of Heroes’ game-core pillars."
As if they knew all along and we didn't.