I had also thought about a stun-add item. Only thing is, it would need to be Legendary or Epic to ensure Mr. Iron couldn't abuse it. The other thing I'm not sure about is if adding an effect to towers would code properly (As the luck mechanic applies to the tower itself so far, so better luck doesn't affect Painting of Solea or Bottle of Mead). Poison Arrow is pretty cool. Possibly give it a slight negative to keep it from being abused (+100% of Tower's Damage over 3 seconds, -50% Base Tower Damage).
Allright Romeo, you get a 100% dmg (including all the crits, multicrits and so on), which essentially doubles all the dmg done. Which would be more powerful then 1-2 Multicrit in late game. And you say -50% dmg is a disadvantage? XD The reason, why neither Froggy or Solara really work is, that they spend too much dmg on the same target, while having a rather low base dmg. In lategame you get like 20+ hits done before the poison went through... so you make a huge overkill. With this item, you have a surefire way to kill bosses later on. Just think about a 500M gold Laser with that item and lots of Multicrit in the backrow (to safe money) of your defense? The bosses come and well, a huge, huge, HUGE dmg boost on the laser..i think that would be way too powerful, considering the basedmg of many towers (and thus the potency of the potions etc...), if you dont like the laser, take the Dark Forge. But there is one important question left: Does the poison stack or overwrite itself? If the poison stacks... well best carry item ever ;) If it doesnt, it still might be way too strong.
-50% Base Damage would render Critical and Damage potions half as effective. You list the Dark Forge as an example. The Dark Forge would lose hundreds off of its Base Damage later in the game. It would basically take a tower that could consistantly kill Normal, Mass and Air, and relegate them exclusively to Boss and Challenge killers. I don't think it'd be as overpowered as you assume.
Well, lets go for the darkforge, it deals 500 dmg base, with all bonuses, it goes to lets say, 5000 damage. Then you add the poison arrow, it would deal 10000 right away. If you add a Multicrit items it might go from 5000 to 6500 (depending on the stats it got). You basically double his friggn damage, no matter how high it already becomes. If you go for a 11 MC forge and then add this item... it would become a 22 MC forge instantly. And that is not overpowered? Consider other towers with inherant poison abilitys. The basedmg is LOW. UTTERLY low. That is because they get a free huge source of damage. Oh and i was pretty sure it stated "-50% damage" and not "-50 % base damage". -50% base damage is too much i agree. Since it would overall deal the same damage, but adding a big chance to leak cos the time the poison takes... -20 to -25 % might be more realistic. [you might noticed, that i never talked about reduced base dmg xD only about -50% damage, which would be less then many items give as a bonus ^^] The important question is still: Does the poison stack or does it overwrite itself? Example: Solara fire stacks, while the toad, as far as i know, refreshes his poison.