The Instant and Sorcery Game

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{Spoiler}I don't really think this card meets the prompt, though I can see where you were going (I think). There is definitely a downside to playing this card multiple times (well, sort of) and quite possibly a steep one at that. The issue though is threefold:

• It ambiguous whether the downside is enough to even make this card a net downside in terms of playing multiple times, let alone a card you don't want to cast multiple of as per the prompt.
• Adding to the point above, life is the kind of resources many are willing to pay heaps of for advantage and especially since this card rounds down the life loss, the lower your life total the more willing you likely are to play multiple copies of this.
• The wording "the number of times you casted this spell this turn" doesn't work. You only cast each spell once, as even if you're using the same card, it's always treated as a new object by the game rules. The wording you'd want is something like "the number of times you casted a spell with the same name as this spell this turn" (which is how I will treat the card for the rest of the feedback/review)

A card like this is difficult to evaluate, not only because of the randomness element combined with potentially extreme damage, but also because of the unusual way to determine it - your remaining life total. Played relatively early and assuming no life gain, the card can deal upwards of 8-9 damage (technically it could theoretically deal 10, but until turn 4 it's likely you'll be dealt at least a little damage), decently reliably and for a price far too cheap for that, but at the same time it's a randomness-based card where only one side benefits you, making it unreliable. Considering the cards of the closest examples I could find to relate to this, mana value-based damage on a top of the deck card, I think how fair the costing here is depends on the interpretation of the card.

Assuming, as the wording appears to suggest, that you flip, then apply effects of the result, then flip again, the card seems pretty well costed. If however, you do all the flips at once and you can pick the order to resolve them, I'd put this at minimum.

{Spoiler}An interesting approach to the prompt, tying the rewards to not playing other spells in general - so, including more copies of this card. Besides, even if you could, you'd discard 0 cards, so you wouldn't get any draws (and things might get confusing that way anyway). I would probably up the cost by considering Change of Fortune, even accounting for all the downsides it has compared to that card.

{Spoiler}Another card that is a little ambiguous in terms of whether it can really fit the prompt, but otherwise an interesting card and one that better shows why it woud be a net negative to play. The problem is the net negative is also present on the original card, and though it may be a useful card in some niche situations, costing three is a massive issue for this card. After all, you don't want delayed benefits if you are also giving up long-term gains. The way I see it, if you removed flashback from Strike it Rich that would practically be a strictly better card, because:

• You get the same mana in total than you lose (your card loses 3 paid now and 1 in every future turn, to to get 2 mana in future turns)
• Treasure comes in untapped
• By not giving up a land, you have the same amount of mana (on net) as given by your card.

For a card like what you made to work it would basically need to be a ritual, or else give considerably greater delayed benefits to justify the long-term loss.

{Spoiler}Meets the prompt well by largely not giving any benefit to playing more of. Technically there still is, because I'm fairly certain if you cast a creature after this resolves, that creature is not affected by this effect. Though there are permanents costing 4-6 giving permanent unblockability, I think a lot of the time the unblockable danger is on a single explosive turn, when it's unblockable on mass at least. Still, you're also massively reducing the damage potential here, making it so the key benefit is mostly on-damage triggers or otherwise in combat tricks. Either way it'll take set-up, and it's technically not full unblockability... I was gonna say add one more mana, but on second thought that may not be necessary after all. I guess it'd need playtesting.

{Spoiler}I'm going to give this victory to @gluestick for creativity in the card and how the prompt was met, as well as a pretty interesting well-balanced card in general. Runner up is @daidjan who had the most fitting card for the prompt and pretty nicely balanced, if just a little off.