I would love a shakeup of the holy trinity with dedicated support classes that don't heal or anything just buff you and debuff the enemy. But that begs the question: How would it work in solo play?
One possible coding nightmare way I can think of is to have the buff be distributed evenly across the party. So for example, if you are alone you get a 50% increase to your main stat. But in a 5-man party, everyone gets 10%. Plus one guy who hits about as well as a healer.