Fabrice made an official post in the Ubisoft forums concerning the Heroes V add-on. It confirms the dwarves race, who will be drawn into war against their will. But fabrice also warns that not all info about the dwarves that has recently leaked into various forums is truthful. Only time will tell us the true details of this nation. Besides the new faction the add-on should feature simultaneous turns, random map generator, snow terrain (judged by the screenshot) and the return of the caravan

The official French Might and Magic site has some additional screenshots. (thanks Eternor for pointing it out)