One of the few ideas EQ2 had which I really liked was cast through which I think would address this problem. If you cast a harmful spell ordid an attack on a party member it was applied to whatever mob that member had targetted as if you had that mob targetted directly. If you cast a beneficial spell on an enemy it would be automatically applied to whoever that enemy had targetted.

This made combat far less of a click fest. In addition to making assisting and healing the main tank a no brainer it also simplified other situations. If my paladin were soloing or the main tank and I wanted to heal while fighting a mob, all I had to do was cast the heal spell. I had the mob targetted but it had me targetted so the heal spell landed on me without me having to change targets.

EQ2 still has a bug whereby if you're fighting through someone else and the mob dies, it doesn't take you out of combat mode and you have to hit ESC to get out. That bug has been in since day one. However, it's not hard to hit ESC and the feature as a whole is quite nice.

I suspect it would take a lot of coding to make it work in HZ though.