The card itself shouldn't be changed in any way, as it serves well its purpose; versatility is its main problem as I see.
The best buff in my opinion for Purify would be something similar that makes Poison so great: that there are other means to apply it - this way it can stack up real quick and pose as a threat for the opponent, even with its negligible 1-3 damage. If Purify-counters would get at least another mean to apply, it would also become a 'threat' for many decks with the continiously growing regen. Of course it shouldn't be spammable as much as poison because that would mean immortality for you after awhile, especially paired up with other sources of healing, but maybe a weapon (or a shield even) would do well, from which you can have only one at a time (unless building a deck around it).