For the DPS side, if they're so desperate to keep Healers only having one attack spell, buff it up to the point where it matches Tanks damage output, or nerf Tanks damage to match healers. Because Tanks aren't supposed to be DPS either so there's no justification at all on why they should be doing more damage than Healers.
And one thing that was suggested in a thread some time ago, replacing most heals from Tanks with shields. That alongside with removing heals from a lot of the attacks on tanks would probably do wonders to the game's design.