I would love to see combo moves. Some ways could be:
extra damage done when a move is done to a player who is on fire/poisoned/bled. (pyromancer, necromancer, ranger, samurai, etc?)
A high cost move that causes DoTs on the target to finish instantly, but do less overall damage
After using an offensive move or perhaps successfully cleansing a debuff, a healer could receive a small boost to non-self healing, to encourage healers to use not just blanket heals.
A move that boosts the damage of the next attack, but increases the cooldown of that attack, depending on the skill. Would have a long cooldown itself.
I also know of a hero server, now long defunct, that had several really cool hitbox skills, such as Backstab, where rogues added a slow to their target when hitting them from behind, or Headshot, which passively added damage to attacks that are aimed at the head. I have heard its hard to code these, but they can add a lot to PvP variation.