My suggestion is to make Mind Control's cost variable, according to the minion's cost (my favorite choice) or category. For example: to cost 2 more mana than the cost of the minion (minimum of 6) , or to cost 8 to control basic, common and rare minions, 9 for epic and 10 for legendary .
In the present scenario, who faces a priest must attract the mind control for less important minions, to play the most powerful ones later. Also, the current meta consists in numerous and very efficient aggro decks , and sometimes the priest has trouble to control the game. With the changes I suggested , the way to play as priest and against it would change a lot, because the Mind Control could be used earlier in the game, in theoretically less important but troubling minions, and still allows the opponent to make use of his most powerful and expensive cards by about two turns before the priest uses Mind Control (keeping the idea proposed by Blizzard) .
PS: Yeah, I know there are other topics about this, but I resolved to post here to give more visibility to the idea.