An idea my girlfriend had for a co-op adventure game was players would create decks that had a variety of abilities.
All ability cards would have an effect activated when put in play and attack values for one to all eqight spots on the cards. These spots would be the four corners and then the middle of the four sides of the card. A strong ability may have high damage but for only 4/8 spots limiting how the player can position the ability on the grid. Another strong ability may have low damage but can be used for any of the 8 spots. Ability effects will also dictate how strong and how many of the spots are filled for the ability.
A monster would be placed in the center of a 3x3 grid. The monster would have a health value and attack value.
Also, each of the 8 spots (same location as the ability cards) would display a "weakness" icon (ranged, melee, magic, etc).
Now, players take turns with a different "leader" each round of play placing 1 ability card anywhere on the 3x3 grid. But they want their ability's attack values to align with the monster so they are doing maximum damage and optimally aligning their ability damage type with a monsters weakness.
Now, after each round of placing cards (after 4 cards are placed in a 4 player game, 3 in a 3 player game, etc) the player that did the most damage is dealt back damage from the monster so you generally dont want your lower health characters dealing out the most damage because they may die.
SO how do you offset who is attacked and for how much then? Characters can equip items that give them bonuses. Like the "tank" of your group can equip a shield at the beginning of the game that gives the +1 emnity. This means that an attack by the tank which deals 3 damage appears to do (3+1) 4 when the monster is determining who to attack.
Play continues until all 8 spots are filled. If the monster has not be defeated once all 8 slots are filled the group all receives a large penalty usually discards and/or life loss.
But besides just attack values, abilities will also offer heals, attack misdirection, protection, absorbs, combo boosts (with adjacent abilities), effects to allow two or more abilities to be played in the same grid spot, etc all to aid the characters in defeating the monster.
Upon success, special equipment and advanced ability cards are drawn from a communal deck to bolster the players' decks.
