You wait for the book to be published and see if they added a class feature (or introduced a feat) to use Constitution instead of Dexterity for AC when wearing light armor.
On another note, Mordenkrad will be 2d6. They removed the oversized rule. So instead of Mordenkrad, you could instead just use a Craghammer or Waraxe, and grab light shield proficiency.
Just for the record, Minotaurs no longer get oversized weapons.
Dragon Magazine 369 gives their full PC write up, with racial feats and everything, and it overrules the limited info in MM.
They loze oversized weapons. They gain a +2 to AC against Opportunity attack caused by a charge and a +2 bonus to perception and nature. The article also includes cool new racial feats, a minotaur paragon path and more fluff on their culture then you get on a regular race in the PHB...
Concerning AC, no, there is no magic solution to increasing your AC if you won't take an heavy armor. Be sure to make good use of the barbarian's ability to regain temp hp.