I like your core pont, at least it appears to be the main point which is that the trinity is breaking down because everyone is measured by their dps and DDs are just not that powerful. That forces everyone into the same mold - DD and DD-lite.

I have a solution in mind. Leave things as they are for solo play, but when in a party make tanks tankier (increase damage resistance and aggro multiplier), and do less DPS (reduce attack potency), make Healers healier (strongerbuffs and heals) and do less DPS (reduce attack potency), make DDs stronger (increase potency of attacks), but squishier (decrease damage/status resistance.

This restores DD to the correct place in a party, while at the same time compensating tanks and healers by strengthening their respective roles. Since this is a buffonly when in a party, it doesn't alter solo capability, but for group play it encourages the trinity and knowing your role.