Card would be way too powerful, but it would be cool if it fed off an opponent's creature instead, like: Enchant creatureWhenever another creature enters the battlefield or dies, put a -1/-1 counter on enchanted creature.When enchanted creature dies, put a 1/1 green Saproling creature token onto the battlefield under your control for each -1/-1 counter on it.
We'd have to change the name to something else, though. Parasite instead of Symbiote, perhaps?
Oh good point, I guess symbiote wouldn't work anymore. I like the card and flavor better though. Maybe something else entirely, not even a fungus?