it already has purely dps classes (both on range and melee) and it also has an almost purely healer class; the advantage of those classes is that if someone wants to focus on a specific role then he can be confident that role will not be totally ruined in a certain class (e.g. it's approximately impossible for locks to be terrible on all their specs or for a priest to be terrible on both healing specs).
One could say they should make a 13th class or a 14th class for whatever reason (this topic's reason or another with another goal); but that's probably a bad idea because the game already has 12 classes; that might make it already overwhelming to a new player in terms of class choice.
For that reason it might be a good idea to reshuffle a current class for goals of this type; for an almost purely tanking class I believe warriors would be fitting to have 2+ tanking specs (member dk of wotlk?) maybe as a 4th spec; warrior is a quintessential tank class since vanilla.