MArcomage

In the last update there was some cards (lighthouse and oracle) that are good only in a hidden game. I suggest to add them another option:
lightouse could have something like "if the game is not hidden then wall +10";
and for oracle: "if the game is not hidden then lowest resource +3"(just random ideas).
Same thing for card useless in hidden game as intrigue: "if the game is hidden then enemy stock -4". and so on for all the discarding cards.
In this way we can use the same deck for both the modes.

Seems unlikely (or very tricky) that the system could take game mode in to account. In the other case it would make some card text length rather lengthy.

My very personal solution:
Don't play Hidden XD

Yeah, there are already several cards that assume that you are able to see what is in your opponent's hand to play them effectively. That being the case, you have the situation already where certain decks are not suited for either game type. I don't think it would be worth the trouble of examining every card and applying extra modes if they're better suited now for one game type over the other.

I agree wholeheartedly with Progressor's solution.

(I actually don't see the appeal in hidden mode at all, beyond the initial novelty. To me it's like the regular game, except with more randomness/less strategy. *prepares to be flamed*)

I propose instead of taking in account the mode of the game, set the condition
"if the selected card was revealed in previous turn then do the alternative"
if the mode is Hidden then this works fine, if the mode is not Hidden then this will allow for the alternative to work every time

and also a personal remark: I kinda like Hidden mode because it gives greater place for strategy. an example: you have to have knowledge for the Holy cards in opponent's hand before playing "Divine Intervention" (player with most Holy cards in hand gains +9 tower + 17 wall). It so happens some times to give the bonus wall and tower to the opponent just because you didn't measure correctly. If the game is not hidden then a mere glance will tell me what the opponent has and will not be as fun as keeping track of his cards.
More examples apply but in short, Hidden game mode provides more versatility and more factors are required to win aside from luck, including memory and brains. In non-hidden modes i just look in my opponent's hand and just just play the best card I have in mine if there are no counters in the opponent's hand. In Hidden modes this can not happen, unless you are focused in the game and keep track of the opponent's cards and keywords and slowly puzzle out his deck by thinking how he must have combined the cards he's playing.

I guess. I don't really consider counting and memorizing your opponent's cards to be a particularly interesting or fun aspect of the game. To me, the strategic aspect of Marcomage comes in trying to predict how your opponent will act and thinking many steps ahead. When you can see your opponent's hand you can play out different scenarios in your head, and plan ahead, thinking of how your opponent might react to your moves and you his. With hidden games, you have to be a lot more short-sighted and make a lot of gut decisions.

Mode system means having deck for each mode. It is unavoidable even if you modify some cards.

I think I agree with xpoh here.

Strategy applied to the different modes is of a completely other type

I still think the easiest solution is a sideboard. Allow X slots that automatically swap depending on hidden or not.
1 slot would probably be OK, where u could have Intrigue / Lighthouse for example. 1-3 slots total, or 1 for each rarity would be the best.
Although the amount of slots should be around 1/5th of total amount of cards specifically for hidden games, if not less.