MArcomage

I tried to use it once, but didn't find it particularly usable.
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But that's not the reason I start this thread. I believe the side effect should be changed for the same reason as Undead. Illusion keyword effect will discard the newly drawn card. In the case of Illusion this can be particularly annoying with cards like Mirror Strike and Mask of the Phantasm. It then even overrides the intended card effect.
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My suggestion would be somewhere in between the Restoration and Destruction effect: If you play a Illusion after an uncommon Illusion You steal a random facility (in line with Illusions like Curse) unless it brings your opponent under 9 facility's. In that case you steal 5 random resources
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Second idea: In line with cards like Mirror Golem, it could give a bonus / attack depending on the rarity of the opponents card in matching position.
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Any thoughts?
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(BTW, how is Corrosion Illusion? Corrosion is a quite natural phenomena.)

I actually kind of love the illusion side effect as it is. I use it all the time in my decks now.

Im not denying it can have some charm, but it's in conflict with card effects. Also, the keyword effect discards. Undead did that too, but is changed for this reason. The reasoning applies on Illusion too.
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Third idea: it randomly picks another keyword effect. So, it might discard an opponents card to Searing fire, or you'll get back 1/3 of the cost, or deal 2/5/10 dmg, depending on second Ilusion's rarity.

I like the illusion keyword effect too but it does conflict with many illusion card effects. Maybe if another card in your hand (non-rare) is replaced by a rare instead of the played card?

That's true about it overriding effects. It's not something I often notice though, because of the way the illusion, nature, destruction, and restoration keywords work. The second card played can be common, but the first can't, so I generally do uncommon -> common, and only one of the commons (Doppelganger) has a card spawning effect, and generally you'd much prefer a rare to the card opposite it.

There is a difference between the Undead and the illusion keyword effects. Undead could discard a card from hand and because of that it was changed, since that is not allowed. But Illusion keyword effect does not discard a card from hand, it only changes what will be the next card drawn, which is OK.

But on the other hand, I like the suggested ideas ;)