Was letting my mind wander and I started mulling over Immune, a mechanic that really excites me whenever I see it pop up even though its rare. Specifically, I got to wondering if rather than requiring Immune to be either temporary, "while attacking," or hero-limited, what if you limited it to specific classes of damage. What follows are my rough card ideas, plus some thinking on how they'd word/could be expanded.
First, spell immunity. I envision this being like an upgraded version of the Elusive/can't be targeted by spells or hero powers ability. Only, now not only can you not target the creature, but even untargetted damage like Magic Missiles or Flamestrike will just be no-sold. Minions and spells with destroy effects would still be useable and deadly.
And if we're doing Immune to spell damage, why not mirror it with immunity to minion damage. In practice, I imagine that a blanket "Immune to minions" would work the same as if the minion had "Immune while attacking minions" and "Immune while being attacked by minions" going at the same time.
Ethereal Drake is also my quick and dirty attempt to show how Immune can be further broken down or remixed. In this case, anything with 4 or less Attack wouldn't be able to hurt it, but a 5 Attack minion would still be able to take it down. You could then go on to give specific tribe Immunities, like "Immune to Beasts" or whatnot. For spell immunity, this could be stuff like "Immune to spells which Cost more" or other esoteric distinctions.
ethereal drake would have to be like a 5 cost 2/2 to be even remotely balanced. there are so many buff cards. it would be positively broken in paladin or druid. conversely, the archmage seems underpowered - or at least, i'm not wholly sure what mage deck would actually want to run a 7 mana 7/7 that resists spell damage.
i'm not sure if these blanket immunities are good design space to explore. but i do really like interactions in the game with dragonfire potion, for instance. more subtype-related modifiers would be interesting.
Immune cards are something I have talked with others about and the issue mostly centers on it ignoring mechanics of the game to make a mechanic.
One example not on me at the moment was a 5 6/3 that 'Ignored Taunt'. That sucks because Taunt is an important method of protection and ignoring such makes the card un-unique, lazy and basically a hard counter. I do see where you are coming from with this but while the Archmage seems somewhat fine, The Drake is terrifying with buffs and instead I would argue 'Minions LARGER then it' to make a 'David/Goliath' moment.
Others I would think would be unique but still cheap would be 'Ignore AoE damage' or 'Ignore non-damaging effects from spells'