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

[Reynhart]

From a healing standpoint I think a lot of my freedom to DPS comes from the scripted nature of a fight, especialy on SCH and AST but also on WHM to an extent.

This is so very true. And probably applies to tank too. You know that you can drop your stance for x amount of time because you'll always have a CD ready for the next big hit, and that, in the mean time, damage is only scrath compared to much the healer can heal you. Which also might show that healers are a bit too powerful.
If you take out enrage mechanics and imagine a healer with infinite MP, the party would survive indefinitely. For me, that's a problem.