I can't figure out what the purpose of a 4th role would be in this game. Dps attack the enemy, tanks are attacked by the enemy and healers allow other roles to continue fighting. I guess another role could be support whose purpose is to provide the resources needed by other roles to cast their abilities, but in order for that to work the roles of the current trinity would have to be unable to sustain their own resources. A complete rebuilding of the combat system doesn't seem likely to happen.