I don't know if something like this was suggested before, but how about making hybrid classes? this also would work for healers. When I said hybrid, I didn't mean red mage-like from past games. But stances.
So a class can be both, dps and healer or dps and tank depending on the stance. Instead of adding different skills for each stance, you would use the same ones on both, but they would have different effects. To balance it more, when switching to tank/healer stance, damage calculation on skills would be lower like any MT/OT or healer atm.