How about making the Venom strength proportionate to the amount of poison counters (with a cap, of course)? So maybe:
1-3 = Venom
4-6 = Deadly Venom
7+ = Triple Venom? (Which is nearly impossible to obtain, it would have practically no health, and is extremely situational anyway, but hey, why not?)
I like that it clears poison counters on something, too. That is a nice touch.