To be precise, I propose a Neutral non-Legendary Core spell that has a very small cost (0 or 1) that is immune to counters and which sends a particular card back into your hand if it gets countered or otherwise prevented from being played onto the table.
As to why? There are certain decks which are completely dependent on a particular Legendary card. A good example in existence right now is Control Warrior, which really begins to do its thing after you play Odyn, who lets your hero have attack power to the amount of Armor you added that turn. Another is Rainbow mage, which similarly depends on Sif.
If Odyn is countered or its effect is otherwise prevented from taking hold when you play it – it’s game over and you must concede. The whole deck’s viability depends on Odyn’s effect, and you basically can’t win if Odyn is counterspelled or some such. Your deck has tons of spells that provide Armor, but that Armor cannot be used for Odyn-provided offense, so you basically have next to no offense.
So, this proposed non-Legendary Neutral Core card would provide a safeguard in a deck such as Control Warrior. First, you play the spell, which can’t be targeted by any counters, then you play Odyn, and if Odyn is countered, the Mana you spent is still wasted but Odyn remains in your hand for another shot.
You’re still not in the clear if this happens, because the opponent now knows what you’ll do next turn AND you just wasted 8 mana plus the cost of the counter-proofer to try and play it. Unless you chose to put the second counter-proofer and have the other counter-proofer on hand, your Odyn can STILL be countered next turn. But such a card will let a person whose deck depends on one card remain in the game instead of having to concede.
Keeping one or two such Neutral cards in the deck comes at the price of, well, having to keep two cards in the deck which take the place of some other useful card. But while I am sure there will be people who won’t want to use this card, it’ll provide a very specific but beneficial purpose which needs to exist in Hearthstone but doesn’t exist yet.