yeah. I don't think mindcontrol is that big of a problem. Not that much more then poly, big game hunter, or anything else in that nature.
Those cards existing make the big legendaries, etc not instant game over as it is. And as it is, you come ready for it. you play a priest, you hold a poly, hex, big game hunter, assassinate, etc, in case he has MC.
and yeah, mana cost change I dont see doing much. Rarity change so it effects arena, maybe. Extra effects like Khalid mentioned, probably a good idea as well. Take control of minion, discard a random card from your hand. or, only works on minions with 50% or lower of their max life.