Here are the stats I would want:
Sp Ability: Solid Rock
With 180 defense and Steel typing theres no question that the guys at GameFreak intended Aggron to be a defensive pokemon that could still pack a punch, it didn't go to well because it also carried a Rock typing which gave it 4X weakness to the popular Fighting Pokemon and the popular EQ. Also with its low Sp defense and HP it couldn't take Special Attacks well either, this lead to people to use Aggron as an Attacking pokemon and even if with Head Smash+Rock Head he could cause big damage it wasn't the best choice to pick as an psychical attacker.
This stat boost in both Defense and Special Defense accompanied with even more defense in the form of Solid Rock is suppose to put Aggron in the Defensive spot it deserved. I know losing Rock Head is a big hit to Aggron's attacking arsenal but Head Smash builds can only get you so far I think.
I am more than ok with this. Aggron has the potential to be the game's best tanking pokemon. The only thing I would add to the above is a more diverse and unpredictable move pool, with the potential for more stat boosting moves and a possibility for chain bred Dragon Dance (like Tyranitar)