"Mitigation in this case was referring to mitigating CC, PC, QC and Hand Manipulation. A creature having DR mitigates the effect of lethal CC. Adrenaline mitigates the effect of most non lethal CC."
ah, i see, but this isn't exactly the protection i had in mind, as adrenaline seems to mostly be an offensive ability that happens to help with delay mechanics.
"One of the concerns to be wary of is how swingy situational benefit can be.
A card that stops your deck from being altered would be very situational since only Reverse Time and Shard of Bravery alter the opponent's deck."
nightmare adds cards to the hand, and silence prevents using cards in the hand, i consider this to be hand altering as well. and i agree, the minor bonuses would have to be on cards with other benefits as well, similar to sundial both preventing attacks and having a draw rate increase incorporated in it, or a creature like saraph that has a minor skill in addition to its good atk/cost ratio.
On the other hand, we all know how common CC is. Anti-CC cards would usually be useful in a match.
"It would be very difficult to balance an immaterial Sanctuary. The difference in value against QC and non QC decks would be too great."
an immaterial sanctuary is already possible with a 2 card combo- but you're right, i would not suggest a card that has a hard negation of itself and other cards or areas (hand, quanta, creatures,permanents,etc) simultaneously within the single card.
"From this I would conclude a guideline that the more situational the anti control, the softer and cheaper it should be."
i dont know if it has to be that linear, i think it could be more flexible, as you can provide other balancing mechanisms or add other incentives to use the card, which will impact the cost of the card.