I also really like the mechanic, and i agree on the fact that it should keep 0 attack, making it immortal to shields, perhaps its an idea to make it a 0|4? thus harder to kill.
It's not that I don't know or don't have a vague idea, but please tell us what a "Will-o'the-wisp" is and what its relation to its suggested ability is.
I do remember that will-o-wisp is a non damaging attack at pokemon, it just burns the target (thus it fits the theme of a non 'direct'damaging card and it would benefit the extra defence (as i proposed) more than some extra attack.
but to look it up for you, wikipedia says this:
'A will-o'-the-wisp or ignis fatuus (Latin, from ignis, "fire" + fatuus, "foolish")—also called will-o'-wisp, corpse candle, and is the folklore term for a ghostly light sometimes seen at night or twilight over bogs, swamps, and marshes. It resembles a flickering lamp and is sometimes said to recede if approached. Much traditional, non-scientific belief surrounds the phenomenon.'
thus it makes sense with the pokemon attack, burns (iginis means fire) and i like the idea that you implented.
Reducing/burning the HP of the opponent whenever he/she draws a card.
It makes a new and really interresting mechanic into the game.