I wish, that's what I wanted to do when I started playing the division. Having better aggro mechanics would be great but the way it is now you're better off just having a full damage team unfortunately.

It can be viable with a good team. But solo play its terrible

Define tanking. If you're talking about having max armor and health to the point that you are extremely "tanky" then yes this is the best way to survive in the hardest game modes. If you're talking about aggro control, ie holding npcs attention so that teammates can use "squishy" high damage builds to kill them, it's viable but it's not very efficient. If you are playing coordinated group play, like you have to be to make this playstyle work, you're better off just having four well rounded players that can deal and take damage in moderation. There's really nothing so dangerous that you need a dedicated tank to kill it. Just have high toughness and decent damage and you'll do just fine.