Taking an extra action every round for the remainder of a scene or until your enemy is taken out is a pretty potent invoke for someone who hasn't spent refresh on it. Probably more potent than I'd generally allow.
The fact that you have to spend your extra action on a weapon 0 attack against a specific enemy is a pretty serious drawback, though.
On that thought, couldn't it work precisely like incite emotion? Grant +2 for maneuvers and let you make blocks based on venom (paralyzed) with fists. Since you can make blocks already, that would not really be that much more. I just like reducing powers to a common denominator.
You could use Incite Effect to represent poison, certainly.
But since Incite Effect doesn't do damage over time, people who want to do damage over time will still need Venomous.
Maybe instead of offering the tag, we could let the poison's strength be equal to the roll made to inflict it. That could get pretty incredibly lethal if the user spends Fate Points on their poison maneuvers, but given how much it costs to do that it's probably okay. Venomous Weaponry costs as much as Evocation, after all.
And maybe we could waive the surcharge for area poisoning. Might as well be generous, if we're dropping the tag.
Okay, how's this look?
NATURAL WEAPONRY [-1]
Description: Your body contains or can produce some kind of weapon or attack.
Note: You have to define what your natural weapons are when you take this power.
Skills Affected: Fists, Weapons, Guns, Discipline
Effects:Natural Weaponry. Your body contains a weapon with a rating of 2. This weapon has a no range, is not capable of spray attacks, benefits from Strength powers, and is wielded with the Fists skill.
Potent Weaponry [-1]. Your natural weapon has a rating of 4.
Summoned Weaponry [-0]. You must take a supplemental action to create or draw your natural weapon before you can use it. In addition, you may wield your natural weapon with the Weapons skill.
Ranged Weaponry [-1]. Your natural weapon has a range of three zones and is capable of spray attacks. It may be wielded with the Fists, Weapons, Guns, or Discipline skill. It cannot be used with the defence trapping of Weapons unless you possess the Summoned Weaponry upgrade.
Area Weaponry [-1]. Your natural weapon may be used to make zonewide attacks.
Selective Weaponry [-1]. (Requires Area Weaponry) You do not harm yourself when making a zonewide attack against your own zone with your natural weapon.
Explosive Weaponry [+1]. (Requires Area Weaponry) Your natural weapon may not be used to make attacks that are not zone-wide.
Imprecise Weaponry [+1]. (Requires Selective Weaponry) You suffer a -2 penalty to the accuracy of any zonewide attack that you make with your natural weapon.
Venomous Weaponry [-2]. You may use your natural weapon to perform special maneuvers. These maneuvers may only inflict one specific Aspect; usually it's POISONED, but it can also be something like ON FIRE or ACCELERATED AGEING if that fits your character better. You may not tag that Aspect, but during your turn in each exchange after the one you performed the special maneuver in each character with that Aspect suffers a weapon 0 physical attack against Endurance with an accuracy equal to the roll you created the Aspect with. You may perform these special maneuvers against every character in a zone, including yourself, if you possess the Area Weaponry upgrade. Outside of a conflict, the effects of these maneuvers are adjudicated by the GM. But as a general rule, they're quickly fatal.
I'm actually fairly happy with this, but it's the middle of the night so that might just be sleep deprivation talking.
Also, I made one or two slight alterations to the Power's wording that having nothing to do with the new upgrade. Just in case you care.