Great Idea! Certainly makes sense and adds a benefit to the spell that would not only benefit the player using it, but place the opponents at a disadvantage as well.
Although a good idea, this certainly is not a need in Grepolis. Simply because there is no need to change the details of the spell because there's nothing wrong with it currently. It's just like some asking to make Zeus's lightning bolt capable of targeting the structure you would like to destroy ( some may find good some may find it bad but that's not the point just need an example ) now that would sound good to most people but the thing is , its not needed as much in the game.
I feel there are far more things that should be implemented prior to this ( like the privilege to cancel an accidental click with full reimbursement for an upgrade of a building or the training of an unit for a short period of time, maybe 15 seconds, then if clicked after the current reimbursement can be applied)
But overall though not a bad idea
