As far as Tank and Healer synergy goes, the better one is at their role, the less the other will need to be.
If you have a tank that mitigates like a god, then you make your healers bored and have less to do with their primary role except... add additional DPS.
If you have a healer that heals like a god(like they currently do now in contrast to most other MMOs), then you make your tanks bored and have less to do with their primary role except... add additional DPS.
Not to mention you also make them more one dimensional and less appealing to the majority of players. i.e For whatever reason; rocks, water, and air hurtling at you at 1000mph heal you(as amusing as that would be) and giant axes and swords bounce off any physical matter, merely somehow agitating the monster more than being shot/stabbed/blown up would. Muh fantasy.
The fact of the matter is that Tanking and Healing are absolutely artificial parameters that ultimately, just prevent loss and thus can be seen as generally unappealing to most. I've said this since day one and that as long as the goal of every fight is to "Defeat X Boss", the true parameter that allows victory is Damage. There is absolutely no reason to do better at Tanking or Healer other than to fulfill a strange one dimensional fantasy because it does nothing for the progress of the fight towards victory. Not to mention considering SE's views and track record, they want to make a game where the required parameters like tanking and healing are easy enough that a monkey could play it.
Buuut... If we're talking about making the primary roles more intriguing WITHOUT taking away their meaningful contribution then you can for example remove all passive cooldowns from tanks and make their tanking primarily oGCD or GCD with a counter effect that does more than their average GCD. I.E For 3 seconds reduce the next attack by 20%, 10 second cooldown(oGCD) or Gain a X HP shield. If shield breaks, deal 600 potency(GCD). Something like that where actively tanking also contributes to fighting.
Healers can be turning overheal to damage or some fishpaste like that idk I don't play healers.