The original thing I wanted to do was simply too complex for paper Magic and available word-space. The original idea involved exiling the card to cast later (a la Foretell), and it gaining counters when you cast instants and sorceries. Each of those counters would reduce the spell's cost to cast by .

Reminds me a bit of surge. Knowing this mechanic is in the format would discourage playing extra cards to avoid enabling your opponent's instants. Meanwhile sorceries like this will almost never get discounted anyway

Seems like a workable mechanic for instants. You'll almost always get a discount of 1, but that's ok, you can just price that in. I agree it's likely to be slightly format warping - it'll impact against rush decks.

And while this usually won't aid sorceries much, a potential after-combat discount of 1 or 2 is always nice; and you are indeed setting up for the very rare "Ok, whoops, we dogpiled and now are casting 8 cost at each other for free" multiplayer situations that will echo down in legend.

So yeah, I like the basic idea of the mechanic.

Mechanics that are dependent on the opponent are usually problematic. I suppose you have seen the often suggested version that is really similar to surge and gets discount on your own spells cast, right?

It wold be interesting to know the reason why you'd prefer this - usually the above-mentioned version can be calibrated fairly well by deciding how much generic mana you allow in the cost.

I'm especially baffled that you went from a mechanic that counts your instants and sorceries to a mechanic that counts your opponent's spells.