Basically this would be my thought process on all the QQ over MC (even though I don't think it's overpowered in higher ranked games) while improving Priests mid/early game.
Considering that Shadow Word Pain/Death handles minions that are <3 and >5 attack it should be obvious to anyone that plays a priest, but maybe not so obvious to people that don't, that anything with 4 attack is a Priests weak spot. Then you add in the fact that Priests have no real "good" class 4 drops (lightspawns suck - incoming silence etc) along with a few other cards that just take up room for later in the game when there are clearly better choices in the neutral category - Temple Enforcer/Cabal Shadow Priest.
So my thought process here is basically this:
1.) Reduce MC to 3 Mana and have it only able to Mind Control minions that have 4 attack
- 3 Mana because 4 would be retarded. You would literally get no use out of the card and would be better off using a neutral of your choice.
- SW:P/D handle anything lower/higher that is a real threat
2.) Reduce the cost of Cabal Shadow Priest to 5 with unchanged stats/abilities (because in reality it's not worth of a 6 drop when you can pick cards like Sunwalker or Temple Enforcer atm)
3.) Reduce the cost of Temple Enforcer to 4
- Change stats to 3/3 and keep the +3 health bonus for other minions / or reduce to 3 mana and 3/3 and only buff for 2 (would be on par with Sun Shattered Cleric at that point)
4.) Reduce Holy Nova to 4 mana cost
- Damage 2/Heal 1
and just a couple crazy ideas that really have nothing to do with this ^ above but more for the fact that the majority of the Priest class cards suck complete donkey @!##**%*.
1. Mana Burn (2) - Destroy one of your opponents mana crystals / draw a card
(opposite of innervate)
2. Priestess of Fortitude (2) 2/2 - Increase all of your minions health by 2
(on par with Flame Imp  1/1 +1 =  2/2 +2)
3. Psychic Horror (3) - Causes target to tremble in horror. Target loses any Divine Shield and Taunt for 1 turn. [cc]
(somewhat on par with frostbolt just removes the damage and adds in extra effects)
4. Dispersion [secret] (7) - Any attack made against your minions or hero will result in one damage for one turn. Draw 3 cards.
(random ideas because why not?)
The reasoning behind this "idea of balance" is that you can basically automatically lose against anyone that has built their deck to completely counter priests by playing 4 attack minions. You aren't able to really do much of anything 80% of the time if someone does that unless you're lucky and you can gain board control early - which is unlikely for any Priest. Along with the Mind Control change I propose the additional changes to Temple Enforcer/Cabal Shadow Priest would open up A LOT of room for minions in the 6/7 category and would ultimately build you a better deck instead of relying on Mind Control to take control of someone elses legendary/epic 8+ cost card.