As i said in a previous Topic the Only Solution that is 100 % viable is to give All Melees Tanking abilities, with an easy aggro system génération, to not stress the player & make the job more enjoyable, by dpsing and tanking at the same time !
Ragnarok Online games has been doing that since 2002 and its a pure benediction since you dont lack Tanks at all in this mmorpg.

Also there is the second solution to bring Mage as Tank , it will not be successful as the ragnarok online system , but still better than the classic heavy armor Tank only system. Some people dont like to play heavy armor toons, and will certainly be interested to have a look on a mage Tank (if there is one available), DPS and Healers will still have queue but less than before , if they bring some "diversity" to the tank job !