How can tanking and healing be made more broadly useful in more "role-specific" ways?

[Reynhart]

Consider a wave-by-wave survival encounter. You don't need to kill everything, or technically even anything, but if you do not, the damage becomes overwhelming, not to mention whatever utilities the enemies are throwing your way or onto their allies.

This is the same mentality as "killing ennemies is the best mitgation" and would probably still end with tanks and healers having to do the most DPS available to quickly kill each wave.