A little early, I'm glad to show you the Fortress alternate upgrades. Once again, evilp was kind enough to share them with us at the Round Table. Here are the names of the upgrades and their special abilities:

Stone Defender: Enraged, Armored, Large Shield, Hold Ground, Prepared Position
Harpooner: Shooter, Harpoon Strike
White Bear Rider: Enraged, Bear Roar
Battlerager: Immunity to Mind Control, Bash, Anti-Giant, Battle Rage
Flame Keeper: Shooter, Caster (Deflect Arrows, Fireball), Immunity to Fire, Mark of Fire, Avenging Flame
Thunder Thane: Teleport, Immunity to Fire, Flame Wave, Flamestrike
Lava Dragon: Large Creature, Elemental, Immunity to Fire, Liquid Flame Breath

A table with all the stats can be found here. The expansion is still a work-in-progress, so expect a lot of changes until the release day.