I don't know the game so well yet to suggest something concrete but some CCGs award player for discarding his card.

Simplest thing that comes to mind is +1 flux but I don't know would it be good for the game.

Suggestions? Ideas?

So the only CCG i played (too much of) was Magic the Gathering... and you were never rewarded for discarding. There were effects/abilities that caused you to discard that may have had desireable effects, but when you were forced to because you had too many cards in your hand there was no reward.

Although in this format, it might not be bad idea to consider giving a player something simple like 1 flux. I wouldn't give any more than that though...

The reward is a new card the following round that can be helpful. For instance if you have a hand of Sylvan, and only a Dwarven Hall in your hand, you can discard the Dwarven Hall in hopes of drawing a Grove. The game has some 'helper' things like that to make the game more fun and flowing.

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Agreed, I often find myself discarding for exactly this reason. If you have full hand, it's quite unlikely that all your cards will adequately address whatever situation you're in. The grove example is dire, but even when winning I've discarded to try and improve my situation.

Of course, that's why we discard cards - to gain space for other cards (better, with lower domain, more desirable at the moment, etc.).

But that's not a reward, that's an OPTION.
Free space is not a reward as it will be a card. What card? We don't know. What if the discarded card would be helpful in 2 rounds? Is it a reward to getting rid of this? I don't think so.

Like I said, I don't know if that would be good for the game but if (for instance) player would receiving +1 flux per discarded card it would add another tactical option to the game for sure.

But if that would be adopted, my main concern is: is there any limit of cards in a deck?

I don't know that the game should encourage discarding by default. That being said, a building or enchantment akin to an Ord Stone which had the ability "Discard a card: player gains 1 flux. This ability can only be used once per turn" would do roughly the same thing. And would be a fun combination with Benefits of a Clear Mind.

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What are the benefits of Clear Mind card?

Benefits of a Clear Mind - EE1 | Uncommon
Enchantment
Enchant World

Whoever has the least cards gains a glory at the end of the round.

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I see...

That would be a bit powerful combo