I haven’t thought about healing yet, but it can be possible.
Like a priest using their mana pool to make a stronger shield, but in effect it weakens their offensive and healing spells, they can use talents to make the shield stronger, do some AOE damage or weaken the shield for more offense or self healing.
A warlock is already quite tanky, sacrifice a tanky pet to sustain themselves and be able to withstand more damage, also like the priest less damage done and so on.
A rogue can get a massive increase to their dodge, they already have feint. Shamans can already use shields, hunters have pets, mages can be like the priest, more use of mana and defenses instead of offense.
Why? Cause there are not enough tanks running around and not everyone wants to use one of the current classes that have a tank spec. I have this guy, but for raiding we now have 6 tanks or something so I decided to go DPS on another class I have never done, but that class has no tank spec.
I would for sure keep tanking if other classes have a tank spec, so instead of adding more classes like the DH, add more roles to each spec and do something different.
Imagine a mage, charges in with their shield going, pops some AOE to get aggro and than freezes them in place before blinking out, turning off their shield and doing some DPS before placing it back on and absorbing more damage. That could even work now, but what if it made you tanky enough to actually be the tank?