How to you propose to 'rethink' a card? You take a card that is underused and make it better. Stability is an Earth themed ability, as it maintains its stat-sum through all circumstances. The Earth element is mainly focused around Defense, and this is an adaptive defense. Any negative effect played on it is countered by a positive buff. Weakness is turned into strength. It is as stable as a rock.
It is by far and large not useless Daxx. If it were useless, it would not have gotten this far. The main use is to grow more powerful when damaged. Though now that I think of it, HP probably should start out on a prime number. If someone throws an icebolt, firebolt, or drain life at it, it would go from, say, a 3/7 into a 6/4, or a 9/1.
The mechanic makes sense. In simple terms: its attack goes up when its HP goes down and vise versa.
As for implementation... the scarab card works around storing changes in the number of scarabs on the field, and buffs HP by that difference at the end of the turn. This card could be based on the same principle.
Well, if the scarab calculator works like that, then it would be simplicity itself to modify it work for my idea since it works on stored values. Store the attack and defense as A and B. Current attack and defense before changes is X and Y. New attack and Defense are C and D
C = X + (B-Y)
D = Y + (A-X)
After changes are made, update all variables to the new stats.