(you have good points SnoWeb)
Balance:
Try 3
|2
? (equivalent to 7q=6
+1card=3
+1card+unique slot)
Why it is redundant, IMHO:
ETG has very few cards right now. As such new mechanics provide abnormally large increases in the diversity of the game. This diversity is the part of the appeal of the game that, in general, increases the breadth of the audience and the duration of their attention. Hence cards that increase diversity directly improve the game.
At this point specialized mechanics like Neurotoxin, in general, probably should be left alone so time can be spent on other ideas that are better at increasing the diversity of the game.
So let me be clear: This is not redundant in the sense that only 1 could exist in the game rather I am trying to describe that another form of
mechanic would be drastically better to the point that suggesting a repeat of a mechanic is more redundant than would be desired. Afterall, if Zanz has already made all the pieces does he really need the suggestion?
To sum up: The number and similarity necessary for a pair of cards to be redundant increases with the number of cards in the game.