This is slightly tangential, but worth saying: The more "unique" something is, the more cards the uniqueness needs for the uniqueness to be judged by itself. This is important.
Imagine if bloodshadow made ONE card that used Void quanta. Just one. Not the pillar or the pend, just one Void card. Just one.
... We would judge that card based on its card. We wouldn't be judging the merits of Void. At best, we could be saying "This card is interesting." And leave it at that.
But bloodshadow made many cards that used Void. (At least I hope he did, I admit I haven't checked too much.) And that told him quite a few things about it.
For example, Void turned out to be quite overpowered as a quanta type. With a normal element, if you don't have enough quanta, you can't cast that card. Simple. But you can cast Void at any given time in any given deck. Void pillars help offset the cost. So you can risk it. I mean, if your opponent is using the mark to splash, or using the mark to fix quanta (grabbows using earth to make getting grabbows easier, for example), you can just unload your entire void hand and not even care.
Void also had slightly less design space than what it was intended to look like, given the fact that it can go into any deck, thus shouldn't be allowed to use any programmed weaknesses into the elements, like
struggling with quanta ramp or
having no CC.
This brings back to this argument easily:
If ONE Void card, Just one, was brought into Cygnia, it would feel really weird. We'd need at least 2, or 3, or 5, or 10 cards that use Void to "justify" the uniqueness of Void.
Some mechanics that are unique can get away with only one card:
Nill Eater, for example. But nill eater isn't that unique, really. It makes a lot of sense. It's not complex. It does what it says it does - reduces minimum deck size. It's immediately obvious why this is a good card. It's unique, sure, but it's unique in a low-key way.
Some mechanics, however, have so much... Fruit, to them, we'll say, that it really does feel weird to only add one card with that mechanic, such as Void. It's an ENTIRE ELEMENT! It should have more than one card.
And then there are mechanics that would need so many new cards that it just becomes slightly silly to implement - Take hybrid quanta. This post pretty much sums it up:
I think the biggest problem with introducing duo cards is the UI changes involved with making them.
It would imply needing many, many, combinations of new cards to effectively make itself a part of the game. And in the current game, the introduction of even one card from all the Duo combinations would swamp out the entire rest of the card pool, so I say not yet.
Maybe these could be added into the game when the card pool gets bigger, and we can make a big project out of it, but as the game currently stands, there's no point in adding 1 or 2 duo cards when mono cards work perfectly fine.
TL;DR: The more unique (Or, I suppose, complex cards), the more grand you make your mechanic, the more cards you need that use that mechanic to justify it having it in the game. Problem is, CIA isn't well equipped for methods like that.
PS: I like how I'm now a standard of weirdness in this forum.