I knew that beyond the four basic attack cards, I wanted something that would change play; I wanted there to be unexpected events, twists, and turns that could snatch victory from the jaws of defeat. I didn't want this to be random, but I wanted there to be a bit of strategy and luck combined. I ended up with two things: trickster cards and when revealed effects.
Trickster cards are kind of like wild cards, but they allow you to do something else on your attack, in addition to attacking. These allow you to do things like improve your hand, get a card you really need, or even to completely flip damage with another player. Sometimes, you know that a particularly good trickster card is out there, and the pressure is on to take a player out before they get a chance to use it. It puts a clock on the game, increasing intensity as you try to out-maneuver your foes.
"When Revealed" are strategic effects you choose to activate. You can play the entire game without ever revealing your character - but you get a perk when you do. It allows you to evade imminent doom, or to strategically move damage to another player (depending on the effect your character has). I have found that these also are a big matter of play style; my wife routinely waits as long as possible to reveal her character (and sometimes never does), while I look for opportunities to reveal, and jump on the first chance I get that makes sense. Neither option seems to be particularly more effective than another, but both are strategic, and both cause other players to adjust their strategies in response.