I don't understand the point of the card. This is to hide parts to ennemies but all they can see is their lands and their borders. And the point is to surprise them. Do you think hidding their border won't actually put something on their mind that something is going on ? Also What if you use the card on their land, they are not able to use the armies there because it is now under fog ? No I don't believe in this idea unless I'm missing something.
The point of the card would be to temporarily apply a thicker fog setting to a specified area than the fog level set in the game settings. This can be used to hide your own presence, or just the number or armies and deployment in your bordering territories. It can be a guise to mask your strength from your enemy or a bluff to your enemy question your strength and possibly delay a heavy attack on yourself until it clears and your defences can be assessed. You could even use it on neutral territory if only to make your enemy suspect you are in one place when in fact in another. The card would of course have no effect on the specific territories an enemy themselves control, nor any affect on the person playing the card as there'd then be little advantage. The card would certainly have strategic use imo.
I'm unconvinced that the spy/surveillance/recon cards should cancel out with fog cards. Those intelligence cards' abilities are to clear the fog within the target area so I suspect they should still supersede a card whose ability is to simply thicken/enhance it. Also, cards currently operate on a basis of dominance and priority over one another. Intelligence cards take effect before deployment and currently have no counter. Introducing a card which nullifies them, and vice versa, could see both cards go to waste just as often as they are put to good use. One card having dominance over another would give a true value to each card instead of both simultaneously having equal and potentially zero value.
I don't see why a uservoice wouldn't hurt if you really believe in the idea and want to see it someday
. I agree it's an idea worthy of implementing, but it's not something I have need for and could certainly live without whilst other areas are in dire need of development right now. I don't mean to discourage the idea from uservoice; it's just I have little hope for it in the immediate future. One day (I hope) warlight will attract new players steadily and retain them, and then will finally expand the development team beyond one man; and uservoice ideas will tumble one by one into the development process. So if you want it, make the uservoice; I have severe doubts it would arrive anytime soon but I have much more confidence that it is still a good idea.