Increasing party size, or release two more tank jobs and two more healing jobs would help the issue. I don't think there necessarily -needs- to be more than twice as many DPS classes as tanks and healers just because thats what a DF requires... I'm pretty sure the general popularity of DPS classes will allow for their roles to always be filled. If anything, it would take away the extreme end on both sides (instant for tank, soulexploding long for dps) and replace it with some form of a medium.