I personally liked the way you had the option of enfeebling an enemy with items in the CoP era. Perhaps re-including items that aren't going to be terribly dreadful to obtain would be a fun thing to introduce again. A wolf enemy resembling Fenrir, and you have to throw a steak to distract him for instance. Who here can honestly say that an alliance of steak tossing fighters would not be fun?
Using the terrain to your advantage would be cool as well. For example, an area where you fight a monster that can only take damage when it is standing in water. So you have to move him around where you want, let him get impacted by the water, unleash all the damage you can, and he flees away from his weakness. Cool stuff.
Another thing I'm a fan of, is the damage specific types of enemies a la A Shattoto Ascension (and that one ahriman in Abyssea - Konschtat, but screw that guy.) A monster that is only weak to skillchain damage? Heck yes. Forcing people to work together and play accordingly...uh oh. Of course all melee attacks would have to do at least 1 damage to trigger a skillchain, but you feel me.