Blood Rage: Mystics of Midgard
Worth itBGG rates it 7.89 from 3,272 ratings. Several BGG posters add it to every game, though others say it adds little.
Needs: Just the base game
Adds: Mystic units (neither warrior nor monster, strength 2, invade at no Rage cost) unlocked by new Clan Upgrades that each give your Mystics a special ability.