Problem: Lack of use for Item Set Skills. Many of them aren't even worth getting.
Examples:
Priest's Mana Rune Stone [Void Set] (If you run out of mana at 55+, you're doing something wrong or the boss won't let you use mana items anyways);
Warden's Unbind [Hunter Set] (Pets are immune to CC effects anyways, no use);
Mage's Stone Scale Blade [Fearless Cloth Set] (Only 1 mage secondary has Earth skills, no other combination has any kind of use for this skill at all); The list goes on...
Suggested Solution: Make these skills more class generic and have additional effects based on the player's secondary class.
Example:
For Priest Primary, this would be the general component of the Item Set Skill that would be retained no matter your secondary class.
Enhanced Defenses: Passive: Decreases the amount of damage you take from mobs by 5%.
However, the skill will have additional effects depending on your secondary to mock the Elite Skill idea without giving up the multiclass functionality.
Additional Effects:
Warrior Secondary: Increases your physical attack by 10% of your physical defense.
Knight Secondary: Upon being hit, there is a 10% chance to heal 1% of your health to nearby allies.
Mage Secondary: If Icewind Blade hits critically, the target will be slowed by 50% for 10 seconds.
Scout Secondary: If your Urgent Heal is used on a group member with less than 25% hp, your next Urgent Heal will increase in power.
Rogue Secondary: Your dodge rate is increased by 3% of your magical defense.
If you have any suggestions or concerns, please feel free to leave a comment. (The numbers I have put there are hypothetical, I don't do RoM math).