What's the point of a tank? A tank is a method of achieving a goal, that goal being to reduce damage taken by the party. The standard method is to redirect as much damage as possible to a player or players with the highest mitigation against that damage. There's also the side issue of incoming damage possibly being so high that other players would lack the health to survive it, regardless of their mitigation.
Are tanks necessary? At first it would seem that yes, they are necessary. Perhaps even inevitable, since a party which seeks to maximize its survivability will eventually figure out the best way to reduce incoming damage and inevitably that will way will seem to be the use of a tank.
But what if the tank wasn't the one causing the mitigation and aggro control?
This new model of tanking won't be happening in WoW, but all the tools are there, albeit in weaker forms. Misdirect, tricks of the trade, and hand of salvation all regulate aggro beyond oneself. Auras and totems can extend armor and resistances to an entire raid. Fortitude and blessing of kings grant health buffs. Divine guardian acts as a raid-wide mitigation cooldown. Intervene helps regulate aggro as well as redirecting damage. An entire priest tree is devoted to preventive bubbles. Mages can slow enemy attacks and casting. The tools are there to make other players into the mitigation distraction.
This sort of indirect tanking seems best suited to either a 'trickster' or a 'commander'. The trickster would deal with reducing enemy damage and causing them to attack in ineffective ways. For example, convincing them to attack players with high health or who are far away (though not too far, the goal isn't to replace tanking with trivial kiting). Or causing them to lose all control entirely and even attack the players' enemies. In contrast the commander would focus on strengthening allies to survive the attacks, but could also help to direct enemy attacks through manipulation of the aggro generated by players. Used in coordination these two could create disoriented, weakened enemies who are less of a threat and strengthened allies able to tackle the still-dangerous dangers.
Would you want to play an indirect tank? I think I'd enjoy it. But maybe more importantly, would DPS mind suddenly being the one getting hit in the face and could they trust the person keeping them alive? That's certainly not a new relationship, after all, a tank must trust the healers, but DPS may not be used to having that sort of interaction, often preferring the more aggressive "heal ffs" management style.
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