So, something like this:
Loose Fireball: Costs 6 Fire Quanta.
2/4
Passive: Fireball. 1 damage is dealt to each opponent creature every turn as long as Loose Fireball is in play.
Passive: Extinguished. If, after 3 turns, the opponent still has no creatures on his side of the field, Loose Fireball is destroyed.
Upgrades into:
Angry Fireball: Costs 6 Fire Quanta.
3/6
Passive: Fireball. 1 damage is dealt to each opponent creature every turn as long as Loose Fireball is in play.
Passive: Extinguished. If, after 3 turns, the opponent still has no creatures on his side of the field, Loose Fireball loses 6 health.
The latter loses 6 health instead of being destroyed, thus allowing health increases to continue its survival.
Still, no player would sanely play this card if there were no monsters on the field.