The expansion well be announced officially tomorrow and will contain a lot of specifics. The announcement won't contain anything, though, as far as I have understood things, to have some surprises left.
So I won't start disclosing lots of info now (but I may go a bit into detail after the announcement when I'm back on Wednesday
)
However, I can tell you that, yes, they will be Dwarves, no there won't be Mammooths, yes, the Hero type will be a RuneMage, yes, the racial skill will be Rune Magic, yes, it will work different from anything known so far in Heroes. I don't know the exact workings myself so far, but it looks like the skill will allow you do cast Rune Magic spells in battle by giving up Wood and Ore. The Rune Magic spells are very different from the other spells. Here is an example for a rune:
Rune of Elemental Immunity - Creature gets immunity to 2 random magical elements untill the end of combat.
Generally it can be said that the Rune spells create pretty interesting effects.