Thanks to magumi-yashi for suggesting this mega!

Mega Regirock would get the ability regenerator, due to the fact that It can rebuild its armor. Its Defense would reach 240, and its sp defense would rest at 140. It would also get a boost to its attack stat and lowered speed. As for its appearance, Regirock would look more defensive with heavy rock armor, and it would get a greataxe made of rock in its right hand.
Stat boosts:
40 defense
40 sp defense
40 attack
-20 speed