Melee, Ranger and Caster describe different play-styles, not roles, they're all still DPS.
A melee attacks from close range.
A ranger attacks from far away.
A caster attacks from far away while standing still.
That is what truly differentiates the DPS "roles". Anything that they bring outside of that is arbitrary and could be altered without really effecting how they fundamentally play.
All existing healers play like 'casters' just like all existing tanks play like 'melee DPS'.
