I know many have mentioned their ideas for new traits, but here are mine:
Lethal: Deal 2x skull damage to poisoned enemies.
Holy Armor: Causes true damage to damage armor first.
Reaper: Gain a soul any time an ally matches skulls.
Grave: Gain 1 to all stats per allied and enemy troop that dies.
Barrier: 50% chance to negate a spell targeted on the troop.
Reflect: 30% chance to negate a negative status effect, then apply it to all enemies.
Unstable: Upon death, deal 5 damage to all enemies and 2 to all allies.
Sap: Skull pairs drain 2 mana that gets transferred to attacker. (0 mana gain if enemy has no mana)
First: Allows your team to go first when defending in PvP.
Magic Blitz: All allies gain 1 magic when invading in PvP.
Backdoor: When matching a skull pair, allows the troop to choose between attacking the first or last troop.
Follower: Gain +2 to all stats if in the last position.
Passive: Restore 2 to all stats if no damage is inflicted on the enemy this turn. (“heal”, not additional)
Morphic: Rotates attack with armor, armor with HP, and HP with attack at the start of each turn. (any 0 value becomes 1)
K.O. Resist: If HP and/or Armor is at or above base value, any attack that will cause death is negated to 1 HP. (Armor would have to be lowered below base value to kill with true damage. Would also help prevent a huge skull pair to take something from max HP or max HP/Armor down to instant kill.)
Support: Give a random ally a random stat every time this troop casts.