The current penalties for magic gone wrong is obvious enough - you waste your mana/runes/lives of your magicians...but as everyone knows there are different levels of wrong that can occur. I'd love to see secondary effects for spells that could randomly occur as a result of failed magic - something entertaining :)
I mean..couldn't a failed invisibility spell read "Your wizards gather their runes and begin casting...something has gone horribly wrong! All of your villager's clothes have been rendered invisible! It will be 14 days before this effect wears off..."
which of course would prompt the message "Your villagers are too embarassed to go to work. (14 Days)"
lol, I know it's not a completely practical thing...but I think it'd be kinda cool :) And it'd give another way to balance magic use in the game.