My previous design was: “Opening Gambit: If this minion has been summoned via Darkfire Sacrifice, give it +1/+2 and Celerity.”
The problem I had with it, which is why I decided to edit, was that it was too specific and too limiting for the archetype. I mean, if this card were really in the game, then Ramp players would NEED to use Darkfire Sacrifice SPECIFICALLY on this creature, in order to make it viable. It is a 2/3 for 4, so they would need to ramp it up with Darkfire Sacrifice, or otherwise they would be wasting a lot of tempo, since players want to be playing proactive 4 drops, and not a 2/3 one.
As of why it would limit the archetype: Ramp Abyss is an archetype that seeks to bring late game creatures earlier in the game, so that they can pressure the opponent faster. The problem is: if you have a creature that interacts specifically with Darkfire Sacrifice, then you are monopolizing it for that creature only, meaning that if you don’t ramp it, then that creature is useless. If I use all of my 3 Darkfire Sacrifices ramping any creature other than Moonlight Seeker, then it becomes a 2/3 4 drop with no abilities, meaning that it would be a dead card, since playing it would be a huge tempo loss, because no one wants to play a 2/3 for 4 that doesn’t impact the board immediately. That alone would kill the whole archetype strategy, since you want to be ramping as much BIG minions as you can.
Example: Unseven is 2/4 for 4 with a strong Dying Wish effect, but it is also a huge tempo loss on curve due to it’s weak body for the cost. In order to get value from it, one MUST make use of it’s Dying Wish. If it gets dispelled, then the player probably have lost the game already, since a 2/4 for 4 means an incredible loss on tempo by itself, even if it has a really good effect (L’Kian is an exception, I guess, since she doesn’t need the actual body to impact the board). And, if you can’t get value from the effect, then… uh, it’s pretty much a useless card.
Then, there comes the other scenario: Say that I have used all of my 3 Darkfire Sacrifices to ramp Moonlight Seeker in order to make it a 3/5 Celerity minion for 2. Oh, 3 3/5 Celerity minions for 2 each? Seems good, right? No. Why? Because this would make the player feel obligated to ramp Moonlight Seeker only, meaning that if he wanted to ramp, say, a Spectral Revenant on turn 5, he would likely NOT do it, since it would make Moonlight Seeker literally useless. Therefore, it would kill the whole archetype strategy, since you want to be accelerating the most number of “fat” creatures you can. Being in a dilemma of which minion to Ramp out is not what the archetype wants, let alone a 4 mana one.
Then, my edited design comes into discussion:
Opening Gambit: If you have sacrificed at least one wraithling via Darkfire Sacrifice since your last turn, give this minion +1/+2 and Celerity.
Why the “wraithling only” and “since your last turn” restrictions, you ask? Well, this card was designed with Lilithe’s BBS in mind, since it is very, very rare to see someone playing Ramp Cassyva. Lilithe naturally interacts better with the Ramp archetype, so does Cassyva with Creep.
As of the second restriction: I think it solves the archetype limiting problem I’ve stated earlier; if the player decides to sacrifice a wraithling on turn 5 in order to ramp a Spectral Revenant, then he could cast a 3/5 Celerity minion for 4 on the next turn without feeling obligated to choose between which minion to ramp, meaning that he wouldn’t need to have a dead card each turn.
The idea is to give some incentive to the archetype in the form of a minion that benefits from Darkfire Sacrifice indirectly, meaning that it would be possible to ramp big creatures out while also having a great tool to support them.
@Edit: I hope this comment can clarify the philosophy behind the card ^^