I also noticed that the creature vs creature aspect was conspicuously absent from elements when I first started playing.
Coming from playing MtG, I both liked and disliked that. I loved how simple and automatic the attack phase was, but I also noticed that creatures with more than about 5 HP were rather wasteful unless specifically combo'd with gravity pull (or later catapult)... Even then decks of those type would often get overwhelmed by higher attack creatures due to raw damage output. MtG's creature combat system helped balance that, but at the cost of a long, and sometimes quite tedious, battle phase.
My goal with this card was to add in creature battling without needing heavy player interaction (which wouldn't be possible anyway under the play-by-mail theme).
Thinking on it more, I do agree that making it too broadly available will have a dramatic impact. Perhaps to a fault. So I am definitely leaning heavily towards making it an element specific card rather than an "other" type card. A small cost boost could help too.
That said, I don't think having a card that can add a creature vs creature aspect is necessarily a bad thing.
For the impact on CC, I fully agree that this card would make a major impact, but I don't think it would reduce the value, even for hard damage based CC. What it would do is increase the value of "soft" CC like stun and freeze, since they would prevent enemies from attacking or counter attacking.
It would mean that CC need not actually kill a creature directly, just weaken it enough for your allies to finish the job, or stop its counterattacking from killing your allies.
In other words, CC spells in all forms should actually become MORE useful since they will help your own creatures survive against tougher opponents. (E.g. fires crimson dragon will get squashed by gravity's colossal dragon unless fire's CC is brought to bear)
It would also make creature healing much more valuable, and some spells (such as adrenaline or momentum) could have more of a double edged sword effect. E.g. adrenaline could end up getting the reciepient killed on its weaker attacks and using momentum could mean that a hard hitter like crimson dragon wouldn't get a chance at killing a potential threat.
Long story short, it would force players to think much more strategically. In my opinion that would be a very good thing.
In any event, a major factor in determining what level of impact we would see is card cost. Since I will almost definitely make this an element specific card, its casting cost will be a major factor in determining how frequently it is seen. Thus, if this cards mechanics seem too dramatic, a high cost will help curtail the impact.
With a very high cost, it would only be seen late in the game against mono or duo type decks that are primarily powered by the element of this cards type.
With a very low cost, it would probably become an extremely common sight since it could be used as a "splash" card in almost any deck and powered with mark or nova setups.
The break off point would be somewhere around 2 to 4 quanta… unless people vote it to be a fire type, in which case the break off would be much higher due to immolation. This situation would be similar for light, dark, and perhaps life as well.
(at the moment, gravity and earth seem to be most popular though)
With that in mind, I'm adding a poll for casting cost.