I had a random idea I thought could solve the issue of Healer feeling not that useful in current content.
Tank Endurance.
All tank would have a Endurance bar, all tank defense abilities would cost a little endurance, Endurance itself has no in-combat Regen nor would tanks have any way to restore their endurance (Outside of one ability I'll get to in a moment)
Endurance would recover slowly out of combat.
Healer's would all have a way to restore tank Endurance.
Under this system, Tanks would be able to continue to self sustain themselves and enjoy being unkillable machines....as long as their Endurance can last.
All tanks would have one way to recover their own Endurance. (Two ways if you count the Echo effect so they can continue to solo older content and such)
Paladin: Passage of Arms (Usual effect): Grants the Paladin a Endurance Regen effect while Maintaining the ability.
Warrior: Restless Rage: Channel effect - Regen Endurance and Beast Gauge (About 10 Beast Gauge per second) while maintaining. If maintained for at least 6 seconds grants a Regen of 200 potency for 15 seconds.
Gunbreaker: Ammo Charge: Channel Effect - Regen Endurance and Ammo (1 Ammo per 2 seconds) while maintaining. If maintained for at least 6 seconds Grants 3 stacks of supercharged Ammo, This effect causes attacks that cost ammo to heal the Gunbreaker for 400 potency and remove a stack each time.
Dark Knight: Last Stance: Channel Effect - Grants a shield for 10% health for 20 seconds every second maintained (stacking to a total of 60% health) and Regen Endurance while Maintaining.
Please note these are just examples really and I'm not really a balance expect or anything.
I've really no idea if this is any good or not but it kind of sounded good to me and seems to find a middle ground maybe Tank and Healers. I could be missing some things though.